Suppress noisy WriteToClosedSessionException errors on session teardown

[Copilot]

Since closeOnFlush() was adopted (#543), in-flight writes (including TLS close_notify frames) are flushed before the session closes. If the underlying channel is already gone, MINA's write queue cleanup throws WriteToClosedSessionException and routes it to exceptionCaught(). Because WriteToClosedSessionException → WriteException → IOException, it was being treated as a real socket error — triggering redundant disconnect() calls or log.error when the QF session was already detached — producing a flood of error-level noise on every disconnect.

Changes

• AbstractIoHandler.exceptionCaught() — intercept WriteToClosedSessionException before the IOException branch; log at DEBUG and return early (no disconnect, no QFJ_RESET_IO_CONNECTOR attribute set, since the session is already gone)
• AcceptorIoHandlerTest — add testWriteToClosedSessionExceptionIsHandledGracefully verifying that setAttribute(QFJ_RESET_IO_CONNECTOR) is not called when this exception is caught

if (realCause instanceof WriteToClosedSessionException) {
    log.debug("Write to closed session ({}): {}", ioSession.getRemoteAddress(), cause.getMessage());
    return;
} else if (realCause instanceof IOException) {
    // ...existing handling
}

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