[typespec-ts] Sync tsconfig.src include with warp exports for multi-client packages

[Copilot]

Multi-client packages failed to build with DIST_MISSING: the generated config/tsconfig.src.*.json files hardcoded include: ["../src/index.ts"], while warp.config.yml exported every client's entry point as a separate subpath (./devCenter → ./src/devCenter/index.ts). Since the root src/index.ts imports the deeper client/api files directly instead of re-exporting the per-client barrels, TypeScript never emitted those barrels to dist, and warp's export validation failed.

Changes

• build-ts-config.ts: New getSrcIncludePaths(exports) derives the include list from the same exports map that drives warp.config.yml — mapping ./src/... → ../src/..., dropping non-.ts values (e.g. package.json), de-duplicating, and always keeping ../src/index.ts as fallback. The four buildTsSrc{Esm,Browser,ReactNative,Cjs}Config builders now take exports.
• index.ts: Threads modularPackageInfo.exports into the buildTsSrc*Config builders in both the initial-generation and update/migration paths, keeping the tsconfig inputs and warp exports in sync.
• Tests: test-next/unit/metadata/ts-config.test.ts covering default fallback, multi-client sync, non-.ts filtering, de-duplication, and per-target propagation.

For the multi-client example in the issue, the emitted configs now include every exported barrel:

// config/tsconfig.src.esm.json
"include": [
  "../src/index.ts",
  "../src/devCenter/index.ts",
  "../src/devCenter/api/index.ts",
  "../src/devBoxes/index.ts",
  "../src/deploymentEnvironments/index.ts",
  "../src/models/index.ts"
]