[BUG] Data-handler removeRule guards on ruleData.getHandle(), leaking cache entries when handle is null at deletion time

- severity: Medium
- files: `shenyu-plugin/shenyu-plugin-redirect/.../RedirectPluginDataHandler.java:49`; `.../shenyu-plugin-rewrite/.../RewritePluginDataHandler.java:49`; `.../shenyu-plugin-param-mapping/.../ParamMappingPluginDataHandler.java:49`; `.../shenyu-plugin-modify-response/.../ModifyResponsePluginDataHandler.java:49`; `.../shenyu-plugin-context-path/.../ContextPathPluginDataHandler.java:63`
- description: Five `removeRule` implementations guard removal with `Optional.ofNullable(ruleData.getHandle()).ifPresent(...)`. If `handle` is null when the rule is deleted (deletion event carries only rule ID, or rule was previously updated to null handle), the cache entry keyed by `selectorId_ruleId` is never removed. `RequestPluginHandler.removeRule` and `GeneralContextPluginDataHandler.removeRule` correctly guard on `ruleData` itself.
- impact: Stale rule-handle objects accumulate in cache indefinitely (memory leak + potential stale config reuse).
- suggested_fix: Guard on `Optional.ofNullable(ruleData)` and always call `removeHandle`.
- confidence: Medium
- related_existing: none

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## Low

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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 the five listed removeRule implementations and compare their null handling with RequestPluginHandler.removeRule and GeneralContextPluginDataHandler.removeRule. Trace how each cache entry is keyed by selectorId_ruleId and confirm that deletion with a null handle still reaches removeHandle. Done means all five handlers remove the cache entry when ruleData exists, regardless of whether its handle is null.

Written by the indexing model from the issue text.