Addresses code-review C1 (test goroutine race) and I2 (CF 524) on PR #2362.
C1: TestRunRestartCycle_SaaSPath_DispatchesViaCPProv invoked runRestartCycle
end-to-end, which spawns `go h.sendRestartContext(...)`. That goroutine
outlived the test, then read db.DB while the next test's setupTestDB wrote
to it — DATA RACE under -race, cascading 30+ failures across the handlers
suite. Refactored: extracted `stopForRestart(ctx, id)` from runRestartCycle
as a pure dispatcher, and rewrote the SaaS-path test to call it directly
(no async goroutine spawned). Added a no-provisioner no-op guard test.
I2: Cloudflare 524 ("origin timed out") now triggers maybeMarkContainerDead
alongside 502/503/504. Same upstream signal — origin agent unresponsive.
Verified `go test -race -count=1 ./internal/handlers/...` green locally.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>