Presubmit job stuck in In Progress after Buildbucket build completes #194452

In #193736 (commit 2c407ee8ca34e4eff0ffd50d32bbb8f0e5cbb4f6), the presubmit dashboard shows Linux windows_android_aot_engine as In Progress, even though the Buildbucket build succeeded ~11 minutes after starting. All 17 other builds in the engine stage finished and are marked Succeeded (including 7 builds that completed later), so the engine stage never transitions to the fusion test stage and Dashboard Checks stays queued indefinitely.

Cocoon processed the STARTED Pub/Sub notification for the build (start_time and build_id are populated on the presubmit_jobs document), but dropped the COMPLETED notification:

• SubscriptionHandler.service acquires a 24-hour cache lock (cache.setIfNotExists) before running the handler. If the first delivery is still in flight when Pub/Sub redelivers the message (for example, while Scheduler._markUnifiedCheckRunConclusion is retrying Firestore transaction contention on presubmit_guards when several builds finish around the same time), the redelivery sees !lockAcquired and returns 200 OK. Pub/Sub treats that 200 OK as a permanent ACK and never redelivers the message if the first request subsequently fails or times out.
• Once a presubmit completion notification is lost, nothing recovers it: VacuumStaleTasks only reconciles post-submit tasks (leaving PresubmitJob.updateFromBuild unused), and RerunFailedJob only allows re-running failed jobs.

To resolve this, we need two changes in flutter/cocoon:

1. Don't return 200 OK from SubscriptionHandler.service while an earlier delivery is still in flight (return a retriable non-2xx status on in-flight lock contention, only persist the 24-hour processed marker after the handler succeeds, and clear the in-flight lock in finally on failure).
2. Reconcile stale In Progress presubmit jobs against Buildbucket (similar to VacuumStaleTasks for post-submit) and pass completed builds through Scheduler.processBuildCompleted so stage transitions and GitHub check runs recover automatically if a Pub/Sub notification is ever missed.