fix(messagequeue): release leases on drained partitions

[behinddwalls]

Summary

Why?

A lease is held forever once acquired: ReleaseLease is only called on shutdown and fair-share shedding, renewal is unconditional, and workers are reconciled from the lease set — so when a partition drains (every message acked and garbage-collected, hence absent from discovery), its holder keeps a goroutine polling an empty partition at ~4-5 queries per poll tick, permanently, plus a lease row and a queue_offsets row that nothing ever deletes. On topics with bounded partition keys (queue names) this is harmless stickiness; on topics with unbounded short-lived keys it is a leak proportional to cumulative traffic: buildsignal partitions per batch ID, and the gateway log and cancel topics partition per request ID — every request ever processed would leave a ghost worker behind. The crash variant is worse: a stale lease on a drained partition is never even stealable, because acquisition only probes discovered partitions.

What?

• Idle-lease release: the discovery tick tracks, per owned partition absent from discovery, when it was first observed drained (a partition with any in-flight, postponed, or unacked message still has stored rows and is always discovered — only fully-consumed partitions qualify). After a grace of 2x LeaseDurationMs (60s at defaults) the subscriber deletes its consumer group's offsets row (while still holding the lease, so no concurrent initialization is possible), releases the lease, and reconciliation stops the worker. If a message arrives later, the partition reappears in discovery and is reacquired like any new partition — Initialize recreates the offsets row, and since drained meant zero stored rows there is nothing to replay.
• Stale-lease purge: the lease tick deletes lease rows not renewed within 10x LeaseDurationMs, covering holders that crashed while owning a drained partition. Deleting a stale row is equivalent to expiry — a concurrent renewal refreshes the row and the age predicate skips it.
• Rebalance integration tests pin Retry.MaxAttempts high: they publish messages they never ack, and dead-lettering mid-test would now drain the partitions and dissolve the lease distribution their assertions wait on.

Test Plan

• ✅ New integration test TestIdleLeaseRelease covers the full lifecycle against real MySQL: consume, wait out GC + grace, assert the lease and offsets rows are gone, then republish to the same partition key and assert delivery resumes through normal discovery.