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/cli/src/ui/components/messages/DiffRenderer.test.tsx

@@ -0,0 +1,117 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { render } from 'ink-testing-library';
+import { DiffRenderer } from './DiffRenderer.js';
+import * as CodeColorizer from '../../utils/CodeColorizer.js';
+import { vi } from 'vitest';
+
+describe('<DiffRenderer />', () => {
+  const mockColorizeCode = vi.spyOn(CodeColorizer, 'colorizeCode');
+
+  beforeEach(() => {
+    mockColorizeCode.mockClear();
+  });
+
+  it('should call colorizeCode with correct language for new file with known extension', () => {
+    const newFileDiffContent = `
+diff --git a/test.py b/test.py
+new file mode 100644
+index 0000000..e69de29
+--- /dev/null
++++ b/test.py
+@@ -0,0 +1 @@
++print("hello world")
+`;
+    render(
+      <DiffRenderer diffContent={newFileDiffContent} filename="test.py" />,
+    );
+    expect(mockColorizeCode).toHaveBeenCalledWith(
+      'print("hello world")',
+      'python',
+    );
+  });
+
+  it('should call colorizeCode with null language for new file with unknown extension', () => {
+    const newFileDiffContent = `
+diff --git a/test.unknown b/test.unknown
+new file mode 100644
+index 0000000..e69de29
+--- /dev/null
++++ b/test.unknown
+@@ -0,0 +1 @@
++some content
+`;
+    render(
+      <DiffRenderer diffContent={newFileDiffContent} filename="test.unknown" />,
+    );
+    expect(mockColorizeCode).toHaveBeenCalledWith('some content', null);
+  });
+
+  it('should call colorizeCode with null language for new file if no filename is provided', () => {
+    const newFileDiffContent = `
+diff --git a/test.txt b/test.txt
+new file mode 100644
+index 0000000..e69de29
+--- /dev/null
++++ b/test.txt
+@@ -0,0 +1 @@
++some text content
+`;
+    render(<DiffRenderer diffContent={newFileDiffContent} />);
+    expect(mockColorizeCode).toHaveBeenCalledWith('some text content', null);
+  });
+
+  it('should render diff content for existing file (not calling colorizeCode directly for the whole block)', () => {
+    const existingFileDiffContent = `
+diff --git a/test.txt b/test.txt
+index 0000001..0000002 100644
+--- a/test.txt
++++ b/test.txt
+@@ -1 +1 @@
+-old line
++new line
+`;
+    const { lastFrame } = render(
+      <DiffRenderer
+        diffContent={existingFileDiffContent}
+        filename="test.txt"
+      />,
+    );
+    // colorizeCode is used internally by the line-by-line rendering, not for the whole block
+    expect(mockColorizeCode).not.toHaveBeenCalledWith(
+      expect.stringContaining('old line'),
+      expect.anything(),
+    );
+    expect(mockColorizeCode).not.toHaveBeenCalledWith(
+      expect.stringContaining('new line'),
+      expect.anything(),
+    );
+    const output = lastFrame();
+    const lines = output!.split('\n');
+    expect(lines[0]).toBe('1 - old line');
+    expect(lines[1]).toBe('1 + new line');
+  });
+
+  it('should handle diff with only header and no changes', () => {
+    const noChangeDiff = `diff --git a/file.txt b/file.txt
+index 1234567..1234567 100644
+--- a/file.txt
++++ b/file.txt
+`;
+    const { lastFrame } = render(
+      <DiffRenderer diffContent={noChangeDiff} filename="file.txt" />,
+    );
+    expect(lastFrame()).toContain('No changes detected');
+    expect(mockColorizeCode).not.toHaveBeenCalled();
+  });
+
+  it('should handle empty diff content', () => {
+    const { lastFrame } = render(<DiffRenderer diffContent="" />);
+    expect(lastFrame()).toContain('No diff content');
+    expect(mockColorizeCode).not.toHaveBeenCalled();
+  });
+});

packages/cli/src/ui/components/messages/DiffRenderer.tsx

@@ -105,6 +105,14 @@ export const DiffRenderer: React.FC<DiffRendererProps> = ({
 
   const parsedLines = parseDiffWithLineNumbers(diffContent);
 
+  if (parsedLines.length === 0) {
+    return (
+      <Box borderStyle="round" borderColor={Colors.SubtleComment} padding={1}>
+        <Text dimColor>No changes detected.</Text>
+      </Box>
+    );
+  }
+
   // Check if the diff represents a new file (only additions and header lines)
   const isNewFile = parsedLines.every(
     (line) =>
@@ -233,16 +241,21 @@ const renderDiffContent = (
 
 const getLanguageFromExtension = (extension: string): string | null => {
   const languageMap: { [key: string]: string } = {
-    '.js': 'javascript',
-    '.ts': 'typescript',
-    '.py': 'python',
-    '.json': 'json',
-    '.css': 'css',
-    '.html': 'html',
-    '.sh': 'bash',
-    '.md': 'markdown',
-    '.yaml': 'yaml',
-    '.yml': 'yaml',
+    js: 'javascript',
+    ts: 'typescript',
+    py: 'python',
+    json: 'json',
+    css: 'css',
+    html: 'html',
+    sh: 'bash',
+    md: 'markdown',
+    yaml: 'yaml',
+    yml: 'yaml',
+    txt: 'plaintext',
+    java: 'java',
+    c: 'c',
+    cpp: 'cpp',
+    rb: 'ruby',
   };
   return languageMap[extension] || null; // Return null if extension not found
 };

packages/cli/src/ui/components/messages/ToolConfirmationMessage.tsx

@@ -52,7 +52,12 @@ export const ToolConfirmationMessage: React.FC<
   if (isEditDetails(confirmationDetails)) {
     // Body content is now the DiffRenderer, passing filename to it
     // The bordered box is removed from here and handled within DiffRenderer
-    bodyContent = <DiffRenderer diffContent={confirmationDetails.fileDiff} />;
+    bodyContent = (
+      <DiffRenderer
+        diffContent={confirmationDetails.fileDiff}
+        filename={confirmationDetails.fileName}
+      />
+    );
 
     question = `Apply this change?`;
     options.push(

packages/cli/src/ui/components/messages/ToolMessage.tsx

@@ -90,7 +90,10 @@ export const ToolMessage: React.FC<ToolMessageProps> = ({
               </Box>
             )}
             {typeof displayableResult === 'object' && (
-              <DiffRenderer diffContent={displayableResult.fileDiff} />
+              <DiffRenderer
+                diffContent={displayableResult.fileDiff}
+                filename={displayableResult.fileName}
+              />
             )}
             {hiddenLines > 0 && (
               <Box>