Improved AWS Bedrock Model Discovery

The model discovery function has been enhanced with better error handling and a more efficient implementation:

• Better error handling: The function now catches and logs errors when attempting to discover available models, falling back gracefully to default Bedrock models
• Refactored helper function: The model search helper U81 is now a pure function that accepts the model list as a parameter, making it more testable and reusable
• Explicit null checking: Added explicit checks for empty or null model lists before attempting to search

Usage impact: Users working with AWS Bedrock will experience more reliable model discovery, with clearer fallback behavior when model listing fails.