feat(firestore-bigquery-export): reinstate the Cloud Tasks write buffer

[cabljac]

Fourth layer of the Cloud Tasks write buffer stack and the behaviour change, per the option D decision on #3031: migrating users keep the failure behaviour and cost profile they have today. Supersedes #3103.

A failed inline write now enqueues onto a new syncBigQuery task queue (5 attempts, 60s minimum backoff, throttled by MAX_DISPATCHES_PER_SECOND) after up to MAX_ENQUEUE_ATTEMPTS in-process attempts, instead of self-healing once and rethrowing to the trigger retry policy. The task handler re-attempts the write and rethrows so Cloud Tasks retries; the tracker parks terminal failures in BACKUP_COLLECTION. The trigger drops its retry policy, and a failed enqueue is logged, published as onError, and dropped, both exactly as the extension does. The function needs roles/cloudtasks.enqueuer. The queue function sets maxInstances to 500 to match the gen1 handler, since the gen2 default of 100 would 429 dispatches and burn attempts.

One deliberate change from the extension: the success event is published after the insert and swallowed on failure, closing a duplicate-row route.

89 tests pass. Not verified live: that the deployed env carries FIREBASE_KIT_INSTANCE_ID and FUNCTION_REGION, that the gen2 task function accepts the SDK's ID token, queue creation with the resolved throttle, and the task size limit on large documents.

Fixes #3031.

[gemini-code-assist]

Code Review

This pull request reinstates the Cloud Tasks write buffer for the Firestore BigQuery Export kit, routing failed inline writes to a new syncBigQuery task queue instead of relying on Eventarc redelivery. Feedback suggests introducing a separate terminalTracker in HandlerContext to be used exclusively on the final Cloud Tasks retry attempt. This prevents transient failures from writing premature or duplicate entries to the backup collection.