Description

This pull request introduces a new feature that allows users to edit the current input prompt in an external editor by pressing Ctrl+E.

This functionality enhances the user experience for fast-agent's interactive prompt, especially when composing complex or multi-line inputs.

Changes Made

• Added a new asynchronous keybinding Ctrl+E to src/mcp_agent/core/enhanced_prompt.py.
• Implemented a helper function get_text_from_editor within the same file that:
  • Determines the user's preferred editor (from $VISUAL, then $EDITOR, with fallbacks to nano or vim).
  • Creates a temporary file with the current prompt buffer's content.
  • Launches the editor using subprocess.run.
  • Reads the modified content from the temporary file after the editor is closed.
  • Updates the prompt buffer with the edited text.
  • Includes error handling for editor not found, editor errors, and temporary file issues.
• Ensured that the editor invocation is run in a separate thread using asyncio.to_thread to prevent blocking the main asyncio event loop.
• Updated the help text in handle_special_commands to include the new Ctrl+E shortcut.

How to Test

1. Run fast-agent in interactive mode (e.g., uv run agent.py).
2. At the prompt, type some text.
3. Press Ctrl+E.
4. Your default system editor (or $VISUAL/$EDITOR) should open with the text from the prompt.
5. Modify the text in the editor, save, and close the editor.
6. The prompt in fast-agent should now display the updated text from the editor.
7. Test with multi-line inputs and various editors if possible.

This change leverages prompt_toolkit's keybinding system and Python's subprocess and tempfile modules.