fix: exclude empty stop parameter from LLM completion params

[devin-ai-integration]

fix: exclude empty stop parameter from LLM completion params

Summary

Fixes issue #4149 where models like GPT-5.1 throw an error: "Unsupported parameter: 'stop' is not supported with this model."

The root cause was that _prepare_completion_params always included stop in the params dict, even when it was an empty list []. Some models (e.g., gpt-5.1) don't support the stop parameter at all, and passing even an empty list causes an API error.

Changes:

• Modified _prepare_completion_params to only include stop when it has values AND is not in additional_drop_params
• Added 4 unit tests to verify the fix and prevent regression

This also ensures the existing retry logic works correctly - when a model rejects stop, the error handler adds "stop" to additional_drop_params, and subsequent calls will now properly exclude it.

Review & Testing Checklist for Human

• Test with a model that doesn't support stop (e.g., gpt-5.1, o1-mini) to verify the error no longer occurs
• Test with a model that DOES support stop words (e.g., gpt-4o) with explicit stop sequences to verify they still work correctly
• Verify the retry logic still works: if a model unexpectedly rejects stop, the retry should succeed

Notes

• Link to Devin run: https://app.devin.ai/sessions/42f14eaf9af647aaadbc83f9df19d9cd
• Requested by: João (joao@crewai.com)
• The fix only affects the LiteLLM path (LLM class), not native providers

[devin-ai-integration]

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