Add model discovery integration tests without managed config

[andy-xu-db]

Summary

• Add Tests-table Cases 13–24, where managed config does not exist, stacked on Add model discovery integration tests with managed config #655.
• Cover 12 named tests and 20 configured/fresh executions across Claude Code and Codex.
• Add only the remaining unmanaged discovery inputs, fixtures, and Claude-owned cache inspection support.
• Validate Claude discovery only after the TUI starts. Unity Gateway never primes, refreshes, writes, or locks Claude's gateway-models.json before launch.

Cases

Cases	Scope	Executions
13–16	saved location and discovery-disabled native fallback	4
17–24	provider/location discovery and disabled fallback, configured + fresh	16

Each test function includes its Tests-table case number in its name (test_case_13_... through test_case_24_...). Together with #655, the stack has 44 numbered discovery executions.

Live result

The fresh restacked run 35144561045 exercised Claude Code 2.1.268 and Codex 0.154.0 against the existing integration workspace.

• Cases 15, 16, and 21–24 pass in all 10 executions: disabling scoped discovery preserves each agent's native picker/catalog and creates no UG-owned scoped catalog.
• Claude Cases 13 and 19 open the native picker and create no gateway-models.json. Both Case 17 executions create the correctly scoped Claude-owned cache, but the picker still renders native models.
• Codex Cases 14, 18, and 20 return the native Codex catalog in all five executions because the workspace backend reports /ai-gateway/codex/v1/models unavailable.
• These remain hard assertions: there are no skips, xfails, retries, or weaker expectations.

Claude Code currently does not forward the scoped custom routing header on its native /v1/models request. The Codex path needs the workspace backend catalog endpoint enabled for provider- and location-scoped requests. The inherited managed signal jobs also stop before pytest on the same M2M HTTP 403 described in #655.

Stack

• Depends on Add model discovery integration tests with managed config #655 (andy/model-discovery-integration-tests), which contains managed-config Cases 1–12.
• Add model discovery integration tests with managed config #655 is based on codex/model-discovery-test-stack.
• No dependency on Refresh Claude model cache before scoped launches #622 and no pre-launch Claude cache refresh.

Verification

• Head: 89817938da804e588b10c8848a0328d3d21cb555.
• tests/test_integration_contract.py: 5 passed.
• This PR adds 20 numbered executions; cumulative discovery collection is 44.
• Ruff, formatting, and diff checks passed.
• No local Databricks profile was selected.