fix: resolve project-wide type errors and stabilize core build

This PR fixes over 1,000 type errors across the monorepo by addressing root causes in `tsconfig.json` configurations and the `@google/gemini-cli-core` build process.

### Key Changes:
- **Upgraded `lib` to `ES2023`** in `packages/test-utils` and `packages/vscode-ide-companion` to support modern features like `Error.cause` and `Object.hasOwn`, aligning them with the root configuration.
- **Removed deprecated `baseUrl`** from `packages/core/tsconfig.json` and converted absolute-style imports in tests to relative imports.
- **Stabilized `@google/gemini-cli-core` build** to ensure `.d.ts` files are correctly generated, resolving `TS6305` errors in dependent packages.
- Verified that all workspaces and top-level test suites now pass `npm run typecheck`.

These changes restore the intended type health of the project and ensure consistent build behavior across different TypeScript versions.

cc @gundermanc
Labels: bot-fix

packages/test-utils/tsconfig.json

@@ -2,7 +2,7 @@
   "extends": "../../tsconfig.json",
   "compilerOptions": {
     "outDir": "dist",
-    "lib": ["DOM", "DOM.Iterable", "ES2021"],
+    "lib": ["DOM", "DOM.Iterable", "ES2023"],
     "composite": true,
     "types": ["node"]
   },