[Bug]: LocalSessionTurnGate.gates grows without bound — Semaphore entries for completed sessions are never evicted #3532

Describe the bug

LocalSessionTurnGate.gates is a ConcurrentHashMap<String, Semaphore> that grows without bound. Every acquire(key) call inserts an entry via computeIfAbsent, but TurnLease.close() only calls semaphore::release and never removes the entry from the map. Every historical session key accumulates indefinitely; the Semaphore objects cannot be garbage-collected.

This is a pure memory leak — mutual-exclusion correctness is not affected.

To Reproduce

1. Create a LocalSessionTurnGate instance
2. Call acquire(key) and lease.close() for many distinct keys
3. Use reflection to inspect the internal gates map size

@Test
void closedLeaseEvictsIdleEntryFromInternalMap() throws Exception {
    LocalSessionTurnGate gate = new LocalSessionTurnGate();
    for (int i = 0; i < 50; i++) {
        TurnLease lease = gate.acquire("session-" + i);
        lease.close();
    }
    Field gatesField = LocalSessionTurnGate.class.getDeclaredField("gates");
    gatesField.setAccessible(true);
    ConcurrentHashMap<?, ?> gatesMap = (ConcurrentHashMap<?, ?>) gatesField.get(gate);

    // Actual: map.size() == 50 (all 50 stale entries remain)
    assertEquals(0, gatesMap.size());
}

Expected behavior

After all leases are closed, the internal gates map should be empty (or contain only keys for sessions still in flight). Idle entries must be evicted to prevent unbounded growth.

Error messages

org.opentest4j.AssertionFailedError: gates map must be empty after all leases are released; found 50 stale entries — unbounded memory leak
Expected :0
Actual   :50

Environment (please complete the following information):

• AgentScope-Java Version: 2.1.0-SNAPSHOT
• Java Version: 17
• OS: macOS

Additional context

The same pattern was already fixed in MemoryFlushMiddleware.drainFlushQueue(), which explicitly returns null to evict idle keys with the comment "idle: evict so the map holds only active keys".

Affected file: agentscope-harness/src/main/java/io/agentscope/harness/agent/gateway/LocalSessionTurnGate.java

Proposed fix: in TurnLease.close(), after releasing the permit, remove the entry when no thread is waiting:

return () -> {
    semaphore.release();
    if (semaphore.getQueueLength() == 0) {
        gates.remove(key, semaphore);
    }
};

gates.remove(key, semaphore) is a conditional remove — it only deletes the entry when the map still holds this exact instance, so a concurrent computeIfAbsent that already inserted a fresh instance is unaffected.