Add draft_gmail_message MCP tool

Add new tool to create Gmail draft messages with optional recipients.
This allows users to save email drafts at any stage of composition
without requiring all fields to be completed.

- Implements Gmail draft API integration
- Makes "to" field optional for flexibility
- Uses GMAIL_COMPOSE_SCOPE for authentication
- Maintains existing project patterns for error handling and logging

core/server.py

@@ -25,6 +25,7 @@ from config.google_config import (
     DRIVE_FILE_SCOPE,
     GMAIL_READONLY_SCOPE,
     GMAIL_SEND_SCOPE,
+    GMAIL_COMPOSE_SCOPE,
     BASE_SCOPES,
     CALENDAR_SCOPES,
     DRIVE_SCOPES,