Add missing multipart test coverage for wire names and optional parts

[Copilot]

Improve spector test cases for payload MultiPart

Completed Tasks:

• Install dependencies and build the repository
• Copy TypeSpec definitions to temp directory
• Read mockapi.ts files to understand expected behavior
• Compare multipart mockapi.ts with existing spec files to identify missing test cases
• Update azureIntegration/multipart.spec.ts with new test cases
• Update integration/multipart.spec.ts with new test cases
• Update azureModularIntegration/payloadMultipart.spec.ts with new test cases
• Update modularIntegration/payloadMultipart.spec.ts with new test cases
• Review azureArmMethodSubscriptionId.spec.ts files - they already match mockapi.ts
• Run integration tests to verify all changes work correctly
• Fix test failures and address code review feedback
• Restore accidentally deleted generated type definition files

Summary of Changes:

New Multipart Test Cases Added:

1. withWireName test - Validates wire name functionality where TypeSpec property names differ from HTTP form field names
2. optionalParts tests (3 scenarios):
   • Test with only id field
   • Test with only profileImage field
   • Test with both id and profileImage fields

Files Updated:

• packages/typespec-ts/test/integration/multipart.spec.ts (RLC)
• packages/typespec-ts/test/azureIntegration/multipart.spec.ts (RLC Azure)
• packages/typespec-ts/test/modularIntegration/payloadMultipart.spec.ts (Modular)
• packages/typespec-ts/test/azureModularIntegration/payloadMultipart.spec.ts (Modular Azure)

Files Restored:

• Reverted accidental deletion of generated type definition files:
  • packages/typespec-ts/test/azureModularIntegration/generated/type/enum/extensible/src/index.d.ts
  • packages/typespec-ts/test/azureModularIntegration/generated/type/enum/fixed/src/index.d.ts
  • packages/typespec-ts/test/azureModularIntegration/generated/type/property/additional-properties/src/index.d.ts

Test Results:
✅ All new tests passing for both RLC and Modular clients
✅ Code formatted and reviewed
✅ Comments added explaining type assertions

Note on azureArmMethodSubscriptionId:
The azureArmMethodSubscriptionId.spec.ts files already exist and fully match the mockapi.ts expectations, so no updates were needed for those files.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve spector case for payload MultiPart</issue_title>
<issue_description>typespec link: https://github.com/microsoft/typespec/blob/6c96b6634864a63ba46639314f6414614884e03f/packages/http-specs/specs/payload/multipart/main.tsp

1. run pnpm install && pnpm build
2. run pnpm copy:typespec under folder packages/typespec-ts.
3. run pnpm generate-tsp-only to generate code.
4. According to temp/specs/payload/multipart/mockapi.ts update case for azureIntegration/multipart.spec.ts, integration/multipart.spec.ts , azureModularIntegration/payloadMultipart.spec.ts and modularIntegration/payloadMultipart.spec.ts.
5. create azureArmMethodSubscriptionId.spec.ts for azureIntegration and azureModularIntegration, then complete the case file according to the temp/specs/azure/resource-manager/method-subscription-id/mockapi.ts
6. Verify that all Integration case tests pass. If there are any issues, please fix them.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve spector case for payload MultiPart #3683

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[v-jiaodi]

@copilot why delete enum/extensible/src/index.d.ts, enum/fixed/src/index.d.ts and property/additional-properties/src/index.d.ts? revert these files change.

[Copilot]

@copilot why delete enum/extensible/src/index.d.ts, enum/fixed/src/index.d.ts and property/additional-properties/src/index.d.ts? revert these files change.

The files were accidentally deleted during the initial commit when I was exploring the repository structure. They have been restored in commit 331be4c.