Propagate managed Claude defaults with model discovery

[andy-xu-db]

Summary

• propagate managed Claude default_model with both Unity Catalog and Model Provider Service discovery
• write every explicitly configured Claude family default for both sources
• preserve MPS target IDs verbatim while retaining UC [1m] behavior
• keep local and automatically discovered family defaults suppressed for families the managed config omits
• preserve model precedence: explicit --model, managed budget recommendation, managed default, then provider fallback
• support Fable-only MPS configurations and relayed MPS private settings

Context

PR #716 intentionally skipped automatic model resolution for managed discovery sources, but its guards also skipped defaults explicitly supplied by Coding Agent Config. This restores the managed-policy values without changing PR #744 picker behavior or unmanaged provider behavior.

Testing

• uv run --frozen pytest tests/test_agent_claude.py tests/test_cli.py tests/test_managed_resolve.py tests/test_databricks.py -q (938 passed)
• Ruff check and format checks for all touched files
• independent implementation, risk, and test reviews: no remaining findings

Screen.Recording.2026-09-18.at.6.17.36.PM.mov