chore(ai): change default model to mistral-tools:7b

Updated the default agent model from gemma2:9b-instruct-q5_K_M to mistral-tools:7b
for improved tool calling support.

Testing Results:
- All 49 tests pass successfully
- Environment tests: 5/5 ✓
- LiteLLM raw tests: 4/4 ✓
- Message format tests: 6/6 ✓
- Agent tests: 7/7 ✓
- API tests: 7/7 ✓
- PydanticAI setup tests: 7/7 ✓
- PydanticAI tools tests: 6/6 ✓
- PydanticAI API tests: 7/7 ✓

The model has been verified to work correctly with:
- Simple completions
- Streaming responses
- Tool calling (calculator, date tools, etc.)
- PydanticAI agent framework
- All API endpoints

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

services/core-ai/src/config.py

@@ -25,7 +25,7 @@ class Settings(BaseSettings):
     ollama_timeout: int = 300  # 5 minutes
 
     # Model Configuration
-    agent_model: str = "gemma2:9b-instruct-q5_K_M"  # Optimized for PydanticAI tool calling
+    agent_model: str = "mistral-tools:7b"  # Optimized for PydanticAI tool calling
 
     # System Prompt Variants
     system_prompt_variant: str = "minimal_agent"  # For simple mode