[BUG] MetaDataCache.obtain selects an arbitrary (non-specificity-ordered) matching wildcard MetaData and permanently caches it

- Severity: Medium
- Location:
`shenyu-plugin/shenyu-plugin-base/src/main/java/org/apache/shenyu/plugin/base/cache/MetaDataCache.java:122-138` (scan `:125-129`, cache `:133-134`)
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Description:
On a `CACHE` miss, `obtain` scans `META_DATA_MAP.values().stream().filter(data -> data.getEnabled() && PathMatchUtils.match(data.getPath(), path)).findFirst()`. `META_DATA_MAP` is a `ConcurrentHashMap`; `values()` iteration order is hash-bucket order, not pattern specificity. When two registered metadata paths both match (e.g. `/api/**` and `/api/users/**` both matching `/api/users/42`), the one returned is whichever the CHM yields first — not the more specific one — and that arbitrary choice is pinned into `CACHE.put`. The choice can flip after an unrelated metadata insert/delete triggers a CHM resize (changing bucket layout). Exact (non-wildcard) metadata is pre-cached directly in `cache()` at line 85, so only overlapping-wildcard configs are exposed.
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Impact:
Non-deterministic / wrong-backend routing when overlapping wildcard metadata exists; cached once chosen.
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Suggested fix:
Sort matching candidates by specificity (longest non-wildcard prefix / `PathPattern` comparator) before `findFirst`.
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Confidence: Medium
- Related existing: #6548/#6549/#6550/PERF-04/05 cover unbounded cache and O(n) miss cost; this is the separate selection-arity correctness issue.

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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 in shenyu-plugin/shenyu-plugin-base/src/main/java/org/apache/shenyu/plugin/base/cache/MetaDataCache.java at obtain(), especially lines 122-138, and trace the wildcard scan and cache write. Done means overlapping wildcard paths consistently select the most specific matching metadata and cache that deterministic choice; compare the behavior with exact metadata handling in cache() around line 85.

Written by the indexing model from the issue text.