Fix: Remove stop parameter for o4-mini model

[devin-ai-integration]

Fix stop parameter not supported with o4-mini model

Description

This PR fixes issue #2661 where attempting to use the o4-mini model raises an exception because the OpenAI API returns an error indicating that the stop parameter is not supported for this model.

The fix modifies the _prepare_completion_params method in the LLM class to check if the model is "o4-mini" and remove the "stop" parameter before making API calls.

Changes

• Added check in _prepare_completion_params to remove the "stop" parameter when using "o4-mini" model
• Added a test to verify the fix works correctly

Testing

• Added test to verify that the "o4-mini" model works correctly without raising an exception
• Note: There's a pytest plugin conflict in the environment (pytest-recording vs pytest-vcr) that prevents running tests locally

Fixes #2661

Link to Devin run: https://app.devin.ai/sessions/1756ea2405544d65a3ecd6a27bf25311
Requested by: Joe Moura (joao@crewai.com)

[devin-ai-integration]

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