[BUG] MqttBootstrapServer shutdown NPE on failed start; start swallows InterruptedException

- severity: Medium
- files: `shenyu-protocol/shenyu-protocol-mqtt/src/main/java/org/apache/shenyu/protocol/mqtt/MqttBootstrapServer.java:57-79`
- description: `start()` catches `InterruptedException` from `bootstrap.bind(...).sync()` and does nothing (no log, no interrupt-flag restore, no cleanup). If bind fails, `future` stays null. `shutdown()` calls `future.channel().close()` → NPE on `future`.
- impact: Failed/incomplete start leads to NPE on shutdown and leaked event-loop groups; interrupt status lost.
- suggested_fix: Null-check `future`/groups in shutdown; on bind failure dispose groups and re-interrupt.
- confidence: Medium
- 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)._