fix: Responses API cancel requests can still end as completed

[sam-saffron-jarvis]

What changed

• Updated responseRun.complete to give pending cancellation precedence over completion.
• When a cancel request has already been recorded, the run now transitions to cancelled, emits response.cancelled, and strips completion-only usage fields from the terminal response payload.
• Added a regression test covering the race where cancelRun() wins just before complete() finalizes the run.

Why this is high-value

This fixes a user-visible correctness bug in the Responses API: a client could successfully POST /cancel and still receive response.completed if generation had already finished but the final completion transition had not run yet. That made cancellation flaky on long-running web/API sessions, left the run snapshot looking successful, and could mislead reconnecting SSE clients.

Validation

• gofmt -w cmd/serve_response_runs.go cmd/serve_response_runs_test.go
• go build ./...
• go test ./...
• Added TestResponseRunCompleteHonorsPendingCancellation to verify cancellation wins over completion.
• git diff --check

[SamSaffron]

Applied to the working tree locally (along with #789/#790/#791 as a combined cancellation-correctness pass); will land in a direct commit.