Generate index files for nested clients (#3734)

* Initial plan

* Add expected blob/index.ts file to demonstrate the fix

Co-authored-by: JialinHuang803 <139532647+JialinHuang803@users.noreply.github.com>

* Smoke test passed with blob/index.ts

Co-authored-by: JialinHuang803 <139532647+JialinHuang803@users.noreply.github.com>

* All tests passing - fix complete

Co-authored-by: JialinHuang803 <139532647+JialinHuang803@users.noreply.github.com>

* Simplify condition check as per code review feedback

Co-authored-by: JialinHuang803 <139532647+JialinHuang803@users.noreply.github.com>

* Fix naming consistency for exported types in blob/index.ts

* Add clarifying comment for subfolder condition check

Co-authored-by: JialinHuang803 <139532647+JialinHuang803@users.noreply.github.com>

* Address PR feedback: revert manual changes and simplify condition

Co-authored-by: JialinHuang803 <139532647+JialinHuang803@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: JialinHuang803 <139532647+JialinHuang803@users.noreply.github.com>

Author: Copilot

packages/typespec-test/test/nestedClient/generated/typespec-ts/src/blob/index.ts

@@ -0,0 +1,5 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+export { BlobClient } from "./blobClient.js";
+export { BlobContext, BlobClientOptionalParams, DownloadOptionalParams } from "./api/index.js";

packages/typespec-ts/src/index.ts

@@ -91,7 +91,11 @@ import { emitLoggerFile } from "./modular/emitLoggerFile.js";
 import { emitTypes } from "./modular/emitModels.js";
 import { existsSync } from "fs";
 import { getModuleExports } from "./modular/buildProjectFiles.js";
-import { getClientHierarchyMap, getRLCClients } from "./utils/clientUtils.js";
+import {
+  getClientHierarchyMap,
+  getRLCClients,
+  getModularClientOptions
+} from "./utils/clientUtils.js";
 import { join } from "path";
 import { loadStaticHelpers } from "./framework/load-static-helpers.js";
 import { packageUsesXmlSerialization } from "./modular/serialization/buildXmlSerializerFunction.js";
@@ -319,7 +323,6 @@ export async function $onEmit(context: EmitContext) {
       }
     );
 
-    const isMultiClients = dpgContext.sdkPackage.clients.length > 1;
     emitTypes(dpgContext, { sourceRoot: modularSourcesRoot });
     buildSubpathIndexFile(modularEmitterOptions, "models", undefined, {
       recursive: true
@@ -353,7 +356,9 @@ export async function $onEmit(context: EmitContext) {
         exportIndex: true,
         interfaceOnly: true
       });
-      if (isMultiClients) {
+      const { subfolder } = getModularClientOptions(subClient);
+      // Generate index file for clients with subfolders (multi-client scenarios and nested clients)
+      if (subfolder) {
         buildSubClientIndexFile(dpgContext, subClient, modularEmitterOptions);
       }
       buildRootIndex(