fix: Snowflake PAT auth and Claude streaming support

[josemaria-vilaplana]

This PR addresses two critical issues with the Snowflake provider:

1. PAT Token Authentication Failure

• Fixed KeyError when using PAT tokens with pat/ prefix
• Now correctly strips pat/ prefix and sets X-Snowflake-Authorization-Token-Type to PROGRAMMATIC_ACCESS_TOKEN instead of hardcoded KEYPAIR_JWT
• Maintains backward compatibility with JWT tokens

2. Claude Model Streaming Failure

• Fixed KeyError: 'created' when streaming with Claude models (sonnet-3.5, sonnet-4-5)
• Implemented custom SnowflakeChatCompletionStreamingHandler that provides default timestamp when 'created' field is missing
• Other models (mistral, llama) continue to work as before

Changes

• litellm/llms/snowflake/chat/transformation.py: Added PAT detection logic and custom streaming handler
• litellm/llms/snowflake/common_utils.py: Added PAT detection logic
• tests/test_litellm/llms/snowflake/chat/test_snowflake_chat_transformation.py: Added comprehensive test coverage for both fixes

Testing

• Added unit tests for JWT authentication (existing behavior)
• Added unit tests for PAT authentication with pat/ prefix
• Added unit tests for streaming with and without 'created' field
• All tests verify backward compatibility

Model Compatibility

Model	Authentication	Streaming	Status
mistral-7b	JWT/PAT	✅ Has created	✅ Fixed
llama-3.3-70b	JWT/PAT	✅ Has created	✅ Fixed
claude-sonnet-3-5	JWT/PAT	❌ No created	✅ Fixed
claude-sonnet-4-5	JWT/PAT	❌ No created	✅ Fixed

[mateo-di]

LGTM!