Support for Watsonx ai model for langchain extension

[barvhaim]

Changes for supporting Watsonx.ai (https://python.langchain.com/v0.2/docs/integrations/llms/ibm_watsonx/)

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[greptile-apps]

Disclaimer: Experimental PR review

PR Summary

This pull request adds support for the Watsonx.ai model in the Langfuse Python SDK, specifically for the Langchain extension. The changes span multiple files to integrate Watsonx.ai functionality into the existing codebase.

Key changes include:

• Added Watsonx.ai model parameter handling in langfuse/callback/langchain.py
• Extended model name extraction for Watsonx.ai in langfuse/extract_model.py
• Modified usage metric conversion in langfuse/utils/__init__.py to support Watsonx.ai format
• Updated sampling logic in langfuse/Sampler.py to handle Watsonx.ai event structures

Important considerations:

• Ensure comprehensive testing for Watsonx.ai integration, including edge cases
• Verify that changes in Sampler.py don't affect existing functionality for other models
• Consider adding explicit type hints and docstring updates for new Watsonx.ai-related code
• Evaluate if any additional error handling or logging is needed for Watsonx.ai-specific scenarios

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