[BUG] BaseDataCache.removeSelectData/removeRuleData mutate live list in place while readers iterate unsynchronized (CME risk)

## Description
`removeSelectData`/`removeRuleData` mutate the live `ArrayList` in place under `synchronized(SELECTOR_MAP)`, while `AbstractShenyuPlugin.execute` streams the same list (`matchSelector`/`matchRule`) without synchronization. Concurrent `removeIf` (structural modification) on an `ArrayList` while another thread streams it can throw `ConcurrentModificationException` and/or skip/duplicate entries. Note `selectorAccept` (the update path) correctly replaces the reference atomically — only the remove path mutates in place.

## Location
```
shenyu-plugin-base/.../cache/BaseDataCache.java:133-140 (removeSelectData), 191-198 (removeRuleData)
readers at :173-174 (obtainSelectorData), AbstractShenyuPlugin.matchSelector/matchRule
```

## Impact
During config sync that deletes selectors/rules, concurrent requests matching that plugin can throw CME -> 500/exception and/or skip/duplicate entries. Both a correctness and a latency issue (errors under load during sync).

## Suggested fix
Make `removeSelectData`/`removeRuleData` replace the reference with a new immutable list (like `selectorAccept` already does), or use `CopyOnWriteArrayList`, so readers never observe in-place mutation.

## Related existing issue(s)
#6351 was a CME in a heartbeat subscriber (`ShenyuClientURIExecutorSubscriber`), a different class/path.

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Start in shenyu-plugin-base/.../cache/BaseDataCache.java by reading removeSelectData, removeRuleData, selectorAccept, and obtainSelectorData, then trace AbstractShenyuPlugin.matchSelector and matchRule. Compare the removal paths with the existing reference-replacement update path. Done means concurrent readers no longer observe in-place structural mutations during selector or rule removal, without the reported CME or skipped entries.

Written by the indexing model from the issue text.