Honor x-ms-client-flatten in Bicep type generator (#12001)

# Description

Implements
[`x-ms-client-flatten`](https://github.com/Azure/autorest/tree/main/docs/extensions#x-ms-client-flatten)
in the Radius Bicep type generator so the ARM `.properties.` envelope no
longer leaks into user-authored Bicep templates.

Before:
```bicep
container.properties.container.image
gateway.properties.url
```

After:
```bicep
container.container.image
gateway.url
```

The annotation is already present on every Radius resource (via
`TrackedResourceRequired` in `typespec/radius/v1/trackedresource.tsp`);
the generator just had not been honoring it.

## Type of change

- This pull request adds or changes features of Radius and has an
approved issue (issue link required).

Fixes: #12000

# Contributor checklist

- [x] Existing tests pass
- [x] New tests added (unit + integration + functional)
- [x] Generated artifacts refreshed
- [N/A] Schema/typespec changes — annotation already existed
- [N/A] Sample/doc Bicep rewrites — deferred to a follow-up; this PR is
backward compatible

# Changes

## Generator (`hack/bicep-types-radius/src/autorest.bicep`)

- `src/type-generator.ts`
- New helpers: `isFlattenSafe`, `flagsForFlattenedChild`,
`expandFlattenedInto`.
- Wire flattening into both `processResourceBody` (resource envelope)
and `parseObjectType` (nested objects).
- Fall back to the nested representation (with a warning via
`logWarning`) when flattening would lose information:
    - The schema uses a discriminator (polymorphic variants).
    - A flattened child name collides with an existing parent property.
- Each flattened child is emitted as `ReadOnly`. `Required`,
`WriteOnly`, `Identifier`, and `DeployTimeConstant` are intentionally
not propagated from the wrapper — the flat alias is unambiguously a
read-side projection of the underlying `properties` payload.

## Tests

- `test/integration/specs/basic/.../spec.json` — extended with:
  - `TestType1` — happy-path flatten on a resource body.
- `TestType2` — polymorphic child, exercises the discriminator fallback.
- `TestType3` — child property names collide with the resource envelope,
exercises the collision fallback.
- Checked-in baselines (`test/integration/generated/basic/...`)
refreshed.
- New programmatic Jest suite `test/flatten/flatten.test.ts` asserts the
contract end-to-end: walks the generated `types.json` and verifies
hoisting, fallback emission, and warning behavior.

## Regenerated Bicep types

Ran `make generate-bicep-types`. Updated:

-
`hack/bicep-types-radius/generated/applications/applications.core/2023-10-01-preview/types.json`
-
`hack/bicep-types-radius/generated/applications/applications.datastores/2023-10-01-preview/types.json`
-
`hack/bicep-types-radius/generated/applications/applications.messaging/2023-10-01-preview/types.json`
-
`hack/bicep-types-radius/generated/radius/radius.core/2025-08-01-preview/types.json`
- `hack/bicep-types-radius/generated/index.json`

Spot-checked: e.g. `Applications.Core/containers` now exposes
`application`, `container`, `connections`, `extensions`, `environment`,
`identity`, `restartPolicy`, `runtimes`, `resources`,
`provisioningState`, `status` directly on the resource body, with no
`ApplicationProperties` / `ContainerProperties` wrapper.

## Functional test


`test/functional-portable/corerp/noncloud/resources/container_flatten_test.go`
+ `testdata/corerp-resources-container-flatten.bicep` —
`Test_Container_Flatten` deploys an Application + Container written
entirely in the flat syntax (no `.properties.`) and validates the RP
resources and the rendered pod in the app namespace. The bicep template
only compiles against the regenerated types, so the test doubles as a
regression guard.

# Compatibility

- Flattening is purely **additive and read-side only**. The wrapper
`properties: { ... }` is preserved as the writable authoring surface —
existing templates that author via `.properties.{ ... }` continue to
work unchanged. Hoisted flat aliases are emitted with the `ReadOnly`
flag, so they can be consumed via `reference('foo').bar` / `output x =
foo.bar` but cannot be assigned from a template body. This is
intentional: it eliminates `.properties.` from read-side expressions
without requiring any breaking authoring-side change.
- Existing sample/doc/test Bicep files in this repo are NOT rewritten in
this PR; that is a deliberately scoped follow-up.


# Deployment-engine dependency (resolved)

The deploy-time half of this feature shipped in
[azure-octo/deployment-engine#583](azure-octo/deployment-engine#583)
(merged), which flattens the JToken stored in the reference value lookup
so flat references like `reference('myCtnr').application` resolve at
deploy time. The `:latest` `ghcr.io/radius-project/deployment-engine`
image picks up the fix automatically.

Validated end-to-end on a local kind cluster prior to merge:
`Test_Container_Flatten` passes.

---------

Signed-off-by: willdavsmith <willdavsmith@gmail.com>
Co-authored-by: Dariusz Porowski <3431813+DariuszPorowski@users.noreply.github.com>

Author: willdavsmith

hack/bicep-types-radius/generated/applications/applications.core/2023-10-01-preview/types.json

@@ -1,25 +1,25 @@
 {
   "resources": {
     "Applications.Core/applications@2023-10-01-preview": {
-      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/62"
+      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/72"
     },
     "Applications.Core/containers@2023-10-01-preview": {
-      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/135"
+      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/164"
     },
     "Applications.Core/environments@2023-10-01-preview": {
-      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/176"
+      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/217"
     },
     "Applications.Core/extenders@2023-10-01-preview": {
-      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/197"
+      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/250"
     },
     "Applications.Core/gateways@2023-10-01-preview": {
-      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/220"
+      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/283"
     },
     "Applications.Core/secretStores@2023-10-01-preview": {
-      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/256"
+      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/319"
     },
     "Applications.Core/volumes@2023-10-01-preview": {
-      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/294"
+      "$ref": "applications/applications.core/2023-10-01-preview/types.json#/357"
     },
     "Applications.Dapr/configurationStores@2023-10-01-preview": {
       "$ref": "applications/applications.dapr/2023-10-01-preview/types.json#/55"
@@ -34,16 +34,16 @@
       "$ref": "applications/applications.dapr/2023-10-01-preview/types.json#/114"
     },
     "Applications.Datastores/mongoDatabases@2023-10-01-preview": {
-      "$ref": "applications/applications.datastores/2023-10-01-preview/types.json#/55"
+      "$ref": "applications/applications.datastores/2023-10-01-preview/types.json#/68"
     },
     "Applications.Datastores/redisCaches@2023-10-01-preview": {
-      "$ref": "applications/applications.datastores/2023-10-01-preview/types.json#/77"
+      "$ref": "applications/applications.datastores/2023-10-01-preview/types.json#/103"
     },
     "Applications.Datastores/sqlDatabases@2023-10-01-preview": {
-      "$ref": "applications/applications.datastores/2023-10-01-preview/types.json#/99"
+      "$ref": "applications/applications.datastores/2023-10-01-preview/types.json#/138"
     },
     "Applications.Messaging/rabbitMQQueues@2023-10-01-preview": {
-      "$ref": "applications/applications.messaging/2023-10-01-preview/types.json#/55"
+      "$ref": "applications/applications.messaging/2023-10-01-preview/types.json#/68"
     },
     "Radius.Compute/containers@2025-08-01-preview": {
       "$ref": "radius/radius.compute/2025-08-01-preview/types.json#/125"
@@ -55,19 +55,19 @@
       "$ref": "radius/radius.compute/2025-08-01-preview/types.json#/193"
     },
     "Radius.Core/applications@2025-08-01-preview": {
-      "$ref": "radius/radius.core/2025-08-01-preview/types.json#/44"
+      "$ref": "radius/radius.core/2025-08-01-preview/types.json#/53"
     },
     "Radius.Core/bicepConfigs@2025-08-01-preview": {
-      "$ref": "radius/radius.core/2025-08-01-preview/types.json#/66"
+      "$ref": "radius/radius.core/2025-08-01-preview/types.json#/86"
     },
     "Radius.Core/environments@2025-08-01-preview": {
-      "$ref": "radius/radius.core/2025-08-01-preview/types.json#/89"
+      "$ref": "radius/radius.core/2025-08-01-preview/types.json#/121"
     },
     "Radius.Core/recipePacks@2025-08-01-preview": {
-      "$ref": "radius/radius.core/2025-08-01-preview/types.json#/111"
+      "$ref": "radius/radius.core/2025-08-01-preview/types.json#/154"
     },
     "Radius.Core/terraformConfigs@2025-08-01-preview": {
-      "$ref": "radius/radius.core/2025-08-01-preview/types.json#/138"
+      "$ref": "radius/radius.core/2025-08-01-preview/types.json#/192"
     },
     "Radius.Data/mySqlDatabases@2025-08-01-preview": {
       "$ref": "radius/radius.data/2025-08-01-preview/types.json#/17"