test: typecheck test files in CI; drop mock-only test; richer chrome mock

- typecheck now also runs tsc over tsconfig.test.json (test files were
  never type-checked before).
- Delete tests/unit/background.test.ts, which only asserted the vitest
  chrome mock's own behavior — every remaining test imports production
  code from src/lib.
- Baseline mock gains runtime.id, storage.session/remove, contextMenus,
  and tabs.sendMessage.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -12,7 +12,7 @@
     "zip": "wxt zip",
     "test": "vitest",
     "test:e2e": "playwright test",
-    "typecheck": "tsc --noEmit"
+    "typecheck": "tsc --noEmit && tsc --noEmit -p tsconfig.test.json"
   },
   "dependencies": {
     "@wxt-dev/module-react": "^1.1.5",