fix: correct namespace user relation batch save bindings(#6869)

[juicewcode]

Fixes #6869
Fix the incorrect foreach item bindings in namespace-user-rel-sqlmap.xml.

The batchSave statement declares namespaceUserRelDO as its foreach item, but its five parameter bindings referenced the unrelated namespacePluginRelDO variable. This would cause MyBatis parameter binding to fail if batchSave is invoked.

Changes

• Replace five namespacePluginRelDO.* bindings with namespaceUserRelDO.*.
• Add a mapper XML regression test to verify:
  • the expected foreach item is declared;
  • all five fields use namespaceUserRelDO;
  • no namespacePluginRelDO binding remains.

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.