fix: force-remove containers on pool cleanup and add ref-counting tests (#653)

Pool.cleanup() previously called resource.Close() which respects reference
counting, leaving reused containers running when the pool shuts down. Now
cleanup bypasses ref counting and force-removes all tracked containers.

Adds 5 tests covering the cleanup API and ref-counting behavior:
- TestResourceCloseRefCounting (integration)
- TestPoolCloseT (integration)
- TestPoolCleanupForceRemovesReusedContainers (integration)
- TestCheckForExistingIncrementsRefCount (unit)
- TestGetWithScopeDoesNotIncrementRefs (unit)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: aeneasr