Allow users to specify a custom session_id when opening a browser session,
rather than always generating a random UUID. Useful for naming sessions
for easier management across multiple tool calls.
- Add session_id: Optional[str] = None parameter
- Validate session_id doesn't already exist before starting browser
- Fall back to uuid4().hex[:12] if not provided
- Add tests for custom session_id and duplicate detection
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
