feat: enhance Google Docs and Drive functionality

- Add insert_doc_image_from_drive() with permission checking
- Add insert_doc_image_url() for URL-based image insertion
- Add gdrive/drive_file_permissions.py with permission utilities
- Add gdrive/drive_helpers.py with Drive utility functions
- Add create_insert_text_segment_request() helper for headers/footers
- Improve error handling to distinguish auth vs validation errors
- Maintain backward compatibility with existing functionality
- Include comprehensive error messages and user guidance

Tested successfully with real Google Workspace integration.

gdrive/drive_helpers.py

@@ -0,0 +1,53 @@
+"""
+Google Drive Helper Functions
+
+Shared utilities for Google Drive operations including permission checking.
+"""
+from typing import List, Dict, Any
+
+
+def check_public_link_permission(permissions: List[Dict[str, Any]]) -> bool:
+    """
+    Check if file has 'anyone with the link' permission.
+    
+    Args:
+        permissions: List of permission objects from Google Drive API
+        
+    Returns:
+        bool: True if file has public link sharing enabled
+    """
+    return any(
+        p.get('type') == 'anyone' and p.get('role') in ['reader', 'writer', 'commenter']
+        for p in permissions
+    )
+
+
+def format_public_sharing_error(file_name: str, file_id: str) -> str:
+    """
+    Format error message for files without public sharing.
+    
+    Args:
+        file_name: Name of the file
+        file_id: Google Drive file ID
+        
+    Returns:
+        str: Formatted error message
+    """
+    return (
+        f"❌ Permission Error: '{file_name}' not shared publicly. "
+        f"Set 'Anyone with the link' → 'Viewer' in Google Drive sharing. "
+        f"File: https://drive.google.com/file/d/{file_id}/view"
+    )
+
+
+def get_drive_image_url(file_id: str) -> str:
+    """
+    Get the correct Drive URL format for publicly shared images.
+    
+    Args:
+        file_id: Google Drive file ID
+        
+    Returns:
+        str: URL for embedding Drive images
+    """
+    return f"https://drive.google.com/uc?export=view&id={file_id}"