The `instanceof Response` guard from #10256 broke two ways:
1. `instanceof` is nominal against `globalThis.Response`, but proxyFetch dispatches
through the npm undici package's fetch, whose Response is a different class — so
valid upstream responses were rejected as contract violations. Replaced with
`isResponseLike()` (instanceof fast path + structural brand/member probe); genuinely
malformed shapes still throw.
2. The thrown error had no `.status`, so it fell through to chatCore's BAD_GATEWAY
default — an internal defect was treated as a flaky provider, cooling the connection
down and retrying forever. It now carries status 500 + `executor_contract_violation`,
registered as request-scoped and terminal (no cooldown, no breaker, no retry).
batch_api.test.ts went from exit 124 (infinite hang, pinning Unit shard 4/4 in every
open PR) to exit 0, 22/22 passing.
Closes#10360
90458a613c