[BUG] DisruptorProviderManage.startup is not idempotent — leaks disruptor + executor on re-entry

- Severity: Medium
- Location:
`shenyu-disruptor/src/main/java/org/apache/shenyu/disruptor/DisruptorProviderManage.java:98-134`
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Description:
`startup()` unconditionally constructs a new `OrderlyExecutor`, a new `Disruptor`, new `QueueConsumer`s, and overwrites `this.provider` without shutting down any prior instance. No guard (`provider != null`) and no shutdown of previously-assigned disruptor/executor. Threads are `daemon=false`.
-
Impact:
Any code path calling `startup()` a second time (reconnect, re-init, retry, test re-entry) leaks the previous Disruptor's non-daemon provider threads plus `OrderlyExecutor`/`SingletonExecutor` threads; prevents JVM exit and grows thread counts.
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Suggested fix:
Guard with `AtomicBoolean started`; on re-entry `if (provider != null) shutdown()` first; make `startup` idempotent.
-
Confidence: Medium
- Related existing: none — #6262 is `shutdown()` not closing executor; this is `startup()` not closing the *previous* instance on re-entry.

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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)._

Start with startup() in shenyu-disruptor/src/main/java/org/apache/shenyu/disruptor/DisruptorProviderManage.java:98-134 and trace how the provider, Disruptor, QueueConsumers, OrderlyExecutor, and SingletonExecutor are created. Exercise startup() twice, then verify the prior instance is shut down, no duplicate non-daemon threads remain, and the JVM can exit; use docs/scan2-2026-08-02/06-medium-tiers.md for the scan context.

Written by the indexing model from the issue text.