improvement(scheduler): drain due schedules in chunks (#4578)

* improvement(scheduler): drain in chunks instead of a single capped claim

Replaces the fixed MAX_CRON_CLAIMS (200) with a chunked drain loop:
claim WORKFLOW_CHUNK_SIZE + JOB_CHUNK_SIZE per iteration, process via
Promise.allSettled, repeat until both claim queries return empty or
MAX_TICK_DURATION_MS elapses. Throughput is no longer bounded by a
static per-tick ceiling; it scales until DB or trigger.dev is the
limit. Per-iteration chunk size still bounds row-lock set and fan-out
concurrency.

Extracts processScheduleItem and processJobItem so the loop body stays
readable. Existing claim semantics (FOR UPDATE SKIP LOCKED, lastQueuedAt
as the claim signal, staleness reclaim) are unchanged.

* improvement(scheduler): skip claim once a queue is exhausted and drop workflowUtils non-null assertion

Addresses Greptile review on PR #4578:
- track per-queue exhaustion when a claim returns fewer than CHUNK_SIZE
  rows; subsequent iterations skip the claim query for that queue. Saves
  one DB round-trip per iteration once one queue drains while the other
  is still working.
- narrow workflowUtils to a local const inside the loop body so the
  schedule processing branch only runs when the import has completed.
  Removes the misleading non-null assertion.

Author: TheodoreSpeaks