Add an end-to-end contract test that walks the whole provider journey as one gate: create provider (node) -> add connection -> sync models -> select in Combo -> Playground -> /v1/models exposure -> call via API key -> visible in Topology.
Every step asserts against the same derived contract identity (published model id / configured prefix / raw node id), so a divergence on any surface fails the suite. Directly guards the bugs tracked from Discussion #8273: compatible-provider model regex drift, /v1/models namespace/UUID incoherence (#8327), and Topology blind to custom providers (#8328/#3198).
The primary journey drives the real App Router route handlers + DB layer in-process against an isolated DATA_DIR, so it runs in CI under test:integration (collected by the top-level tests/integration/*.test.ts glob) as a blocking gate with no live server. A second, opt-in block runs the same journey over HTTP and self-skips unless RUN_CONTRACT_INT=1 (same convention as the RUN_SERVICES_INT suites).
Refs #8273. Reported-by: @nguyenha935
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