Fix(diff): Hide whitespace changes in diffs with content changes

- Updated the diff generation in `edit.ts` and `write-file.ts` to include the `ignoreWhitespace: true` option.
- This ensures that whitespace-only changes are not highlighted in the diff output when there are other content modifications, making the diffs cleaner and easier to review.
- Extract default diffing options into single source of truth.

Fixes https://github.com/google-gemini/gemini-cli/issues/548

packages/server/src/tools/diffOptions.ts

@@ -0,0 +1,12 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import * as Diff from 'diff';
+
+export const DEFAULT_DIFF_OPTIONS: Diff.PatchOptions = {
+  context: 3,
+  ignoreWhitespace: true,
+};