Support managed Unity Catalog model locations

[andy-xu-db]

🥞 Stacked PR

• stack/codex/persist-model-location [Files changed]
  • stack/codex/managed-model-location [Files changed] ← this PR
    • stack/codex/claude-picker-ownership [Files changed]

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Summary

• Honor workspace-managed Unity Catalog model locations for Claude Code and Codex configuration and launch.
• Make the admin source override saved developer providers/locations without persisting managed policy into developer state, and reject conflicting explicit source flags before writes.
• Ensure a managed Claude location activates native discovery, enforces Claude 2.1.248 or newer before configuration/launch, and remains free of stale global model pins.
• Keep unsupported agents on their existing discovery path.

Scope boundary

• Depends only on Persist per-agent model locations #673; this model-location stack is based on main.
• It does not define, read, or transport UG_ENABLE_MODEL_DISCOVERY.
• Claude performs native discovery and owns its post-start cache; Unity Gateway does not prime gateway-models.json.
• Refresh Claude model cache before scoped launches #622 is not an ancestor and is not part of this stack.

Verification

• Head: 64e02bfa27b3d95259f3a41d1e91e124f6c2107f.
• tests/test_cli.py at this boundary: 436 passed.
• Location-stack top: 2,540 passed and 42 skipped locally, excluding the native-binary capture test whose two environment-dependent failures reproduce on main.
• Independent risk review: SHIP.
• Ruff, formatting, and git diff --check passed.