fix: Ensure filename is available for diff rendering in write-file

This commit resolves a bug where the `write-file` operation could fail to render content due to a missing filename.

The fix involves:
- Ensuring `fileName` is consistently passed to `DiffRenderer.tsx` through `ToolConfirmationMessage.tsx`, `ToolMessage.tsx`, and `useGeminiStream.ts`.
- Modifying `edit.ts` and `write-file.ts` to include `fileName` in the `FileDiff` object.
- Expanding the `FileDiff` interface in `tools.ts` to include `fileName`.

Additionally, this commit enhances the diff rendering by:
- Adding syntax highlighting based on file extension in `DiffRenderer.tsx`.
- Adding more language mappings to `getLanguageFromExtension` in `DiffRenderer.tsx`.
- Added lots of tests for all the above.

Fixes https://b.corp.google.com/issues/418125982

packages/server/package.json

@@ -12,6 +12,7 @@
     "format": "prettier --write .",
     "test": "vitest run",
     "test:ci": "vitest run --reporter=junit --outputFile=junit.xml",
+    "typecheck": "tsc --noEmit",
     "prerelease:version": "node ../../scripts/bind_package_version.js",
     "prerelease:deps": "node ../../scripts/bind_package_dependencies.js",
     "prepack": "npm run build"