[AIGTWY-4565] Enable Claude MPS model discovery

[andy-xu-db]

Summary

• Enable Claude model discovery for direct and relayed MPS launches.
• Scope /v1/models with the existing Databricks-Model-Provider-Service custom header.
• Keep discovery launch-scoped and remove stale system.ai.* model defaults from MPS launches.
• Preserve explicit provider model pins, including Bedrock.

Server counterpart: https://github.com/databricks-eng/universe/pull/2523045

Usage

Pass both flags before --:

uv run ug claude \
  --provider <catalog>.<schema>.<model-provider-service> \
  --enable-model-discovery

Testing

• Focused: 407 passed
• Full suite: 2191 passed, 37 skipped; 2 unrelated installed-binary E2E failures
• Ruff and diff checks passed
• LiteSwap /model: exactly claude-fable-5, claude-haiku-4-5, and claude-opus-5
• Selected Haiku 4.5 and received E2E_OK; LiteSwap logged the MPS route and HTTP 200

[lilly-luo]

pls wait to merge until backend is fully released. or have a graceful way to handle if someone tries to use this and the backend doesn't have your functionality yet