Openrouter free minimax_m3 hit 400 error while m2.7 gets 200 ok

**Describe the bug**

Using the free open router ai model minimax-m3 resulted in HTTP 400 bad request error. However, changing to m2.7 works just fine which return 200 OK.

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**To Reproduce**
Steps to reproduce the behavior:
1. Install Goose on Mac OS
2. Set LLM provider to Openrouter and configure API key
3. Set model to minimax/minimax-m3:free
4. Say 'Hi' to start new chat.
5. Return error Ran into this error: Request failed: Bad request (400): Provider returned error.

Please retry if you think this is a transient or recoverable error.

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**Expected behavior**
Work just like m2.7 would, get 200 OK and continue.

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**Screenshots**

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**Please provide the following information**
- **OS & Arch:** Mac OS Intel i7 26.6.2 (25G83)
- **Interface:** Desktop UI
- **Version:** v1.49.0
- **Extensions enabled:** Default Installation, no additional extension enabled
- **Provider & Model:** Openrouter GMICloud minimax/minimax-m3:free

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Look at the Openrouter provider integration in the codebase, likely in a directory like src/providers/openrouter.rs. Compare the request parameters for minimax-m3 and m2.7 models to identify what causes the 400 error. Check the provider's API documentation for any differences in required fields or model naming. Run the application locally with debug logging to see the exact request being sent.

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