test: add comprehensive SolidQueueAdapter specs (#409)

40 examples covering constructor guards, interface compliance, enqueue
(auto-fund and finalize paths), idempotent duplicate handling, status
queries, worker success/failure/recovery paths, drain/shutdown, and
thread safety. All specs tagged :postgres, use real PostgresStore, and
mock the broadcaster to keep tests fast (poll_interval: 0.1s).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sgbett