ci: publish Python SDKs to PyPI with OIDC

[IxayWolf]

Summary

• replace the Speakeasy reusable PyPI publish workflow with a local publish job that uses GitHub OIDC trusted publishing
• keep the same publish triggers and package set: mistralai, mistralai-azure, and mistralai-gcp
• add uv exclude-newer = "7 days" to the root, Azure, and GCP projects

PyPI registry setup still needed

• configure trusted publishers for the three PyPI projects with owner mistralai, repository client-python, workflow sdk_publish_mistralai_sdk.yaml, and environment public-sdk

Verification

• git diff --check
• uv build for root, Azure, and GCP packages

After merging

REMOVE THE PYPI TOKEN FROM THE CI AND INVALIDATE IT 🙏