fix(net): fix RejectedExecutionException during shutdown trxHandlePool

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What does this PR do?

Fix RejectedExecutionException during node shutdown in TransactionsMsgHandler.

• Correct shutdown order (core): stop producer smartContractExecutor first, then consumer trxHandlePool, guaranteeing trxHandlePool is alive while the scheduler runs.
• isClosed flag: early exit in processMessage() at entry and during iteration. Covers in-flight messages in the narrow window between PeerManager.close() and handler close.
• RejectedExecutionException catch: handles TOCTOU race between isClosed check and submit().
• Queue cleanup: clear smartContractQueue and queue after both pools terminate.

Why are these changes required?

The original close() shut down the consumer pool (trxHandlePool) before the producer scheduler (smartContractExecutor). During this window, handleSmartContract() was still draining smartContractQueue and calling submit(trxHandlePool, ...) on an already-terminated pool, throwing RejectedExecutionException and polluting shutdown logs.

Thread model

P2P threads ── processMessage() ──┬── submit(trxHandlePool)        [normal txs]
                                  └── smartContractQueue.offer()    [smart contracts]

smartContractExecutor (single-thread, 20ms delay)
    └── handleSmartContract() → take() → submit(trxHandlePool)

close()
    1. isClosed = true
    2. shutdownAndAwaitTermination(smartContractExecutor)  ← producer first
    3. shutdownAndAwaitTermination(trxHandlePool)          ← consumer second
    4. smartContractQueue.clear(); queue.clear()

This PR has been tested by:

• Unit Tests
• Manual Testing: Restart the node repeatedly, verified via log grep:
  • No RejectedExecutionException
  • Correct shutdown order (contract-msg-handler shutdown done before trx-msg-handler shutdown done)

Follow up

N/A

Extra details