Add 9 tests covering the full zombie process prevention chain:
- TestZombieReproduction: demonstrates that processes survive when
references are dropped without explicit cleanup (the original bug)
- TestAgentCloseMethod: verifies close() calls all cleanup functions,
is idempotent, propagates to children, and continues cleanup even
when individual steps fail
- TestGatewayCleanupWiring: verifies stop() calls close() and that
_evict_cached_agent() does NOT call close() (since it's also used
for non-destructive cache refreshes)
- TestDelegationCleanup: calls the real _run_single_child function and
verifies close() is called on the child agent
Ref: #7131
