add AIHubMix as a new Provider

[xinrui-z]

Add AIHubMix as a new Provider, including:

1. Implementation of AihubmixModel class.
2. Example script aihubmix_model_example.py in examples/models.
3. Updated .env.example with AIHubMix environment variables.

Fixes #3369

Checklist

• [x] I have read the CONTRIBUTION guide (required)
• [x] I have linked this PR to an issue using the Development section on the right sidebar or by adding Fixes #issue-number in the PR description (required)
• [x] I have checked if any dependencies need to be added or updated in pyproject.toml and uv lock
• [x] I have updated the tests accordingly (required for a bug fix or a new feature)
• [x] I have updated the documentation if needed:
• [x] I have added examples if this is a new feature

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[fengju0213]

@xinrui-z hi，thanks for this pr! notice that https://github.com/camel-ai/camel/pull/3338 has been merged ,Is this PR somewhat repetitive?

[xinrui-z]

@xinrui-z hi，thanks for this pr! notice that #3338 has been merged ,Is this PR somewhat repetitive?

@fengju0213 hi, thanks for the feedback! Yes, I noticed #3338 was merged. Since then, we’ve made some further updates — mainly adding multi-client routing (now supporting multiple endpoints). I’ve also modified and extended the test cases to cover these changes.