Windows CI flakes on HostSessionClient deny latency assertion #8950

Summary

A Windows root-test shard intermittently fails the HostSessionClient confirm auto-deny test even though the correct deny frame reaches the wire. The test currently treats completion within 50 ms as the contract, which makes correctness depend on shared-runner scheduling latency.

Reproduction

Run:

bun test packages/senpi-task/src/runners/rpc-host/session-client-records.test.ts

In a loaded Windows CI shard, the test #then a deny answer is on the wire within 50 ms observed the correct frame after 55.2205 ms and failed only the elapsed-time assertion.

Expected

• A confirm UI request is answered with confirmed: false.
• The auto-answer path does not wait for a human-facing timeout or block subsequent session commands.
• The test asserts this ordering contract deterministically, without a wall-clock latency budget.

Actual

The payload assertion passes, but the test fails when shared-runner scheduling pushes observed elapsed time above 50 ms.

Evidence

• packages/senpi-task/src/runners/rpc-host/session-client-records.test.ts:42-55 measures performance.now() around socket delivery and asserts < 50.
• packages/senpi-task/src/runners/rpc-host/session-client.ts:206-214 builds the safe deny/cancel response and dispatches sendExtensionUIResponse without awaiting it; there is no timer on the product path.
• Windows CI observed Received: 55.220500000003085 after receiving { type: "extension_ui_response", id: "ui-1", confirmed: false }.
• A 50-run focused macOS control passed, consistent with scheduler-dependent timing rather than a protocol defect.

Root cause

Confirmed test flake: the assertion measures end-to-end wall-clock latency across the socket and CI scheduler instead of the product contract. The product path already dispatches the deny immediately and does not await a UI timeout.

Scope

• Replace the elapsed-time assertion with a deterministic ordering/non-blocking assertion.
• Keep the product path unchanged unless additional runtime evidence reveals an accidental await or timer.
• Prove the fixed test with three focused Windows soak runs of 10 iterations each.

Acceptance criteria

• The confirm request produces the expected deny frame.
• A subsequent session command completes without waiting on the UI response path.
• No fixed sleep, polling delay, enlarged timeout, retry, or platform skip is introduced.
• Three focused Windows soaks pass 10/10 iterations.

Related

• Refs ci(windows): dev push CI fails a rotating Windows test by timeout on every run, blocking the publish gate #8324 for the broader rotating Windows CI failure context; this issue tracks the specific deterministic test repair.