Fixes #11464.

Summary

• preserve dotted ZAI/GLM model ids for opaque custom anthropic-compatible proxies by inferring dot-preserving vendors from the configured model when provider/base_url do not reveal the upstream
• keep Claude hyphenation unchanged on the same custom-proxy path
• add regression tests for both behaviors

Testing

• pytest -q -o addopts= tests/agent/test_minimax_provider.py