fix: Handle Schema import compatibility with vertexai SDK versions

- Add fallback import for Schema from different module locations
- Ensure compatibility with latest vertexai package versions

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: smirk-dev

backend/services/gemini_service.py

@@ -10,11 +10,14 @@
     GenerationConfig,
     Tool,
     FunctionDeclaration,
-    Schema,
     Type,
     GoogleSearchRetrieval,
     Part,
 )
+try:
+    from vertexai.generative_models import Schema
+except ImportError:
+    from vertexai.generative_models.types import Schema
 from typing import Dict, List, Any, Optional, Callable
 import json
 import asyncio