[BUG] meta_data.pathExistedExclude ignores namespace_id → cross-namespace false-positive path collision

- severity: Medium
- files: `shenyu-admin/src/main/resources/mappers/meta-data-sqlmap.xml:154-163`; caller `shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/MetaDataServiceImpl.java:301`
- description: `pathExisted` (line 146) correctly scopes by `namespace_id`, but `pathExistedExclude` (used on update) does `WHERE path = #{path} AND id NOT IN (...)` with NO `namespace_id` filter (verified). So when updating a meta_data whose path also exists in a *different* namespace, this returns non-null and the service raises `DATA_PATH_IS_EXIST`, blocking a legitimate update.
- impact: False "path already exists" errors on meta_data update when the path is legitimately unique within the target namespace but present elsewhere.
- suggested_fix: Add `AND namespace_id = #{namespaceId}` to `pathExistedExclude` and pass namespaceId from the service.
- confidence: High
- 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)._

Start with shenyu-admin/src/main/resources/mappers/meta-data-sqlmap.xml:146-163 and compare pathExisted with pathExistedExclude, then trace the update caller at shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/MetaDataServiceImpl.java:301. Done means updates no longer report DATA_PATH_IS_EXIST when the same path exists only in another namespace, while same-namespace collisions remain detected.

Written by the indexing model from the issue text.