feat(firestore-bigquery-export): add the sync task enqueue module

[cabljac]

Second layer of the Cloud Tasks write buffer stack. Adds src/tasks.ts with enqueueSyncTask and syncQueuePath, and exports SerializedDocumentChange from the handlers as the task payload type. Nothing calls the module yet; the write path is wired in a later layer.

The queue path is deliberately unprefixed so firebase-admin resolves the kit instance prefix. The region comes from the CLI-set FUNCTION_REGION first, with DATABASE_REGION mapped through the dual-region table as the fallback for local runs, and a missing region throws. Enqueue retries in-process with exponential backoff and jitter, capped at 5s, and clamps the attempt budget to at least one. The task id is derived from the event id, so a retried enqueue of an event already in Cloud Tasks returns functions/task-already-exists, which is treated as success rather than buffering the event twice.

tests/tasks.test.ts pins all of the above with a mocked firebase-admin/functions. 75 tests pass. No live queue has been exercised.

Part of #3031.

[gemini-code-assist]

Code Review

This pull request introduces a new tasks module to handle enqueuing Firestore document changes to a BigQuery sync task queue with exponential backoff and jitter, along with exporting the SerializedDocumentChange interface and adding comprehensive tests. The review feedback suggests randomizing the jitter per retry attempt rather than once per task to prevent thundering herd issues, and highlights that passing Infinity for maxAttempts silently defaults to no retries due to a strict integer check.