Remove Claude model discovery proxy

[andy-xu-db]

Summary

• Remove the local Anthropic model-discovery proxy and its client-side model alias rewriting.
• Send native Claude model discovery and inference directly to AI Gateway.
• Preserve the separate subscription-relay token refresh proxy.

Dependency

Merge and deploy the server-side model prefixing/stripping change before releasing this ucode change: https://github.com/databricks-eng/universe/pull/2535560

Testing

• uv run pytest tests/test_agent_claude.py tests/test_claude_smart_routing_v2.py -q — 149 passed.
• uv run ruff check . and changed-file formatting checks passed.
• Full suite: 2,159 passed and 37 skipped. Two binary user-agent tests fail identically on unmodified origin/main because machine-managed Claude/Codex settings bypass their temporary capture server.