[typespec-ts] Fix missing config/ tsconfig files when updating existing packages

[maorleger]

Problem

Since the 0.53.0 release (which included #3938), SDK Validation is broken for all TypeSpec-based PRs in azure-rest-api-specs. The error looks like:

[warp] Invalid config in warp.config.yml: target "browser" references tsconfig
"./config/tsconfig.src.browser.json" which does not exist at
.../azure-sdk-for-js/sdk/cognitiveservices/arm-cognitiveservices/config/tsconfig.src.browser.json

Root Cause

There are two code paths in packages/typespec-ts/src/index.ts for metadata generation:

1. New package (shouldGenerateMetadata = true) — generates warp.config.yml AND the config/ tsconfig files ✅
2. Update existing package (shouldGenerateMetadata = false, i.e. package.json already exists and generate-metadata is not explicitly true) — generates warp.config.yml but not the config/ tsconfig files or tsconfig.json ❌

SDK automation always hits path (2) since the packages already exist in azure-sdk-for-js. So warp.config.yml gets written with ./config/tsconfig.src.browser.json references, but the config/ directory is never created. Maybe it assumes all existing packages in the repo would have been migrated already but that ended up not being the case?

Fix

I added the missing buildTsConfig, buildTsSrcEsmConfig, buildTsSrcBrowserConfig, buildTsSrcReactNativeConfig, and buildTsSrcCjsConfig builders to the update path, matching what the new-package path already does. Also added a consistency test to make sure every tsconfig path referenced in warp.config.yml has a corresponding builder.

Validation

I repro'd the bug e2e with a minimal ARM TypeSpec spec with azure-sdk-for-js: true and a pre-existing package.json (to trigger the update path), then verified the fix by applying the same process:

Before fix:

File	Status
warp.config.yml	✅ Generated (references ./config/tsconfig.src.*.json)
config/tsconfig.src.browser.json	❌ Missing
config/tsconfig.src.esm.json	❌ Missing
config/tsconfig.src.cjs.json	❌ Missing
config/tsconfig.src.react-native.json	❌ Missing
tsconfig.json	❌ Not updated

After fix:

File	Status
warp.config.yml	✅ Generated
config/tsconfig.src.browser.json	✅ Generated
config/tsconfig.src.esm.json	✅ Generated
config/tsconfig.src.cjs.json	✅ Generated
config/tsconfig.src.react-native.json	✅ Generated
tsconfig.json	✅ Updated with ./config/ references

All existing tests pass (rlc-common 103/103, typespec-ts unit tests unchanged). I also confirmed the autorest.typescript generator does not have this issue — it uses a single linear code path with no new/update split.