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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