[BUG] No cleanup on channel close → repository leak for ungraceful disconnect

- severity: Medium
- files: `shenyu-protocol/shenyu-protocol-mqtt/src/main/java/org/apache/shenyu/protocol/mqtt/MqttTransportHandler.java:42-44`; `MqttTransportServerInitializer.java:44`
- description: `MqttTransportServerInitializer` registers `handler` as a `GenericFutureListener` on `ch.closeFuture()`, but `MqttTransportHandler.operationComplete(Future)` has an empty body. Cleanup of `ChannelRepository`/`SubscribeRepository` only happens via explicit `DISCONNECT`. A client that drops TCP without DISCONNECT leaves its `Channel` and subscriptions registered forever.
- impact: Channel and subscription maps grow unbounded across disconnecting clients; future `send()`s try to write to closed channels.
- suggested_fix: In `operationComplete`, remove the closing channel from `ChannelRepository` and `SubscribeRepository`.
- confidence: High
- related_existing: none

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/06-medium-tiers.md`](docs/scan2-2026-08-02/06-medium-tiers.md)._