Allow .ts extension imports in eng typecheck

Running emitter-diff with plain node requires .ts import specifiers, but
the repo-wide check:eng gate (tsconfig.eng.json, --noEmit) rejected them.
Enable allowImportingTsExtensions there so the tool stays covered by the
existing typecheck. Also reword the ensureDeps comment: http-client-python
commits its own package-lock.json to the repo (lockfiles aren't in the
published tarball), which is why npm ci works on a source checkout.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: l0lawrence

eng/emitter-diff/src/adapters/python.ts

@@ -56,7 +56,8 @@ async function isBuilt(dir: string): Promise<boolean> {
  * Ensure the package's npm dependencies are installed. A fresh source checkout
  * (a github clone or a bare local path) has no `node_modules`, so `npm run
  * build` would fail to resolve its types/deps. http-client-python is
- * npm-managed (it ships its own package-lock.json), so `npm ci` here is safe.
+ * npm-managed and commits its own package-lock.json to the repo, so a source
+ * checkout already has the lockfile that `npm ci` needs.
  */
 async function ensureDeps(dir: string, ctx: AdapterContext): Promise<void> {
   if (existsSync(join(dir, "node_modules"))) return;

tsconfig.eng.json

@@ -1,5 +1,8 @@
 {
   "extends": "./tsconfig.base.json",
+  "compilerOptions": {
+    "allowImportingTsExtensions": true
+  },
   "include": ["eng"],
   "exclude": ["eng/vitest.config.ts"]
 }