Fix undefined on multi-file apply_diff output

[daniel-lxs]

With this PR the output should look like this:

Input: { 
  args: `<file>
    <path>src/components/Button.tsx</path>
    <diff>...</diff>
  </file>
  <file>
    <path>src/components/Input.tsx</path>
    <diff>...</diff>
  </file>`
}
Output: [apply_diff for 'src/components/Button.tsx' and 1 more file]

---

Important

Fixes apply_diff output in presentAssistantMessage.ts to correctly display multi-file diffs.

• Behavior:
  • Updates apply_diff case in presentAssistantMessage() to handle multi-file diffs.
  • Extracts first file path from args and counts total files, adjusting output message accordingly.
  • Handles both legacy single-file and new multi-file formats.
• Misc:
  • Adds logic to return a default message if no path or args are provided.

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