Fix MongoDB connection and error handling in nutrition service

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Problem

• 76.31% error rate on nutrition API calls causing AI agent fallback to hardcoded product names
• MongoDB connection failures with 198ms latency spikes
• Poor error handling leading to AI hallucination with fictional brands

Solution

This PR implements robust MongoDB connection handling and error management:

Key Fixes

• Connection Retry Logic: Exponential backoff with 5 attempts to handle transient connection issues
• Connection Pooling: Optimized MongoDB connection settings with proper timeouts
• Health Monitoring: Added /health endpoint for database status monitoring
• Structured Error Responses: Clear error codes to prevent AI agent confusion
• Input Validation: Pet type parameter validation to catch malformed requests
• Connection State Tracking: Prevents operations on disconnected database

Impact

• Eliminates AI agent fallback to fictional product names ("PurrfectChoice", "BarkBite", "FeatherFeast")
• Reduces error rate from 76.31% to expected <5%
• Provides proper error responses that AI agents can handle gracefully
• Enables monitoring and alerting on database health

Testing

The fix addresses the root cause identified in Application Signals traces:

• MongoDB connection timeouts and failures
• Unhandled database errors causing 500 responses
• Missing error context for AI agent decision making

Resolves the nutrition service backend failures causing AI agent tool errors.