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/src/tools/write-file.ts

@@ -202,7 +202,7 @@ export class WriteFileTool extends BaseTool<WriteFileToolParams, ToolResult> {
         ? `Successfully created and wrote to new file: ${params.file_path}`
         : `Successfully overwrote file: ${params.file_path}`;
 
-      const displayResult: FileDiff = { fileDiff };
+      const displayResult: FileDiff = { fileDiff, fileName };
 
       return {
         llmContent: llmSuccessMessage,