fix: scope discovery update and delete by namespace (#6827)

[wy471x]

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

Summary

Changes:

1. discovery-sqlmap.xml — added AND namespace_id = #{namespaceId, jdbcType=VARCHAR} to the WHERE clauses of update (:217), updateSelective (:248) and delete (:254), so discovery mutations are scoped to the caller's namespace; delete now takes (id, namespaceId) parameters.
2. DiscoveryMapper.java:134 — delete(String id) changed to delete(@Param("id") String id, @Param("namespaceId") String namespaceId).
3. DiscoveryServiceImpl.java:192-206 — delete accepts namespaceId and verifies the selected DiscoveryDO belongs to the requested namespace before invoking the discovery processor, so cross-namespace ids produce no registry side effects (also fixes a potential NPE when the id does not exist); the mapper delete is then called with the scoped predicate.
4. SelectorServiceImpl.java:332 / ProxySelectorServiceImpl.java:184 — internal cleanup paths pass discoveryDO.getNamespaceId() (the DO is freshly loaded from DB, so the value is authoritative).
5. DiscoveryController.java — DELETE /discovery/{discoveryId} now requires a namespaceId request parameter (validated via @Existed(NamespaceMapper)); added @RequiresPermissions("system:plugin:edit") to insertOrUpdate and @RequiresPermissions("system:plugin:delete") to delete, consistent with the Selector/Rule controllers.

Test Cases:

• DiscoveryMapperTest — H2 integration tests: delete/update/updateSelective with a mismatched namespace mutate 0 rows; with a matching namespace they mutate only the target row and leave the other namespace's row intact.
• DiscoveryServiceImplTest — delete succeeds in the matching namespace; throws ShenyuException with no processor/mapper side effects on namespace mismatch or when the discovery does not exist.

Verification

• ./mvnw clean install -Dmaven.javadoc.skip=true passed locally (JDK 21).
• Targeted tests: 23 run, 0 failures (DiscoveryMapperTest, DiscoveryServiceImplTest, SelectorServiceTest, ProxySelectorServiceTest).
• checkstyle:check passed.

Note: the dashboard frontend (apache/shenyu-dashboard) currently calls DELETE /discovery/{id} without a namespaceId; a follow-up in that repository is needed to pass the current namespace.

close #6827

related pr: #596