MDEV-38721: add MTR coverage for mhnsw_ef_search bounds

[luvjeetk]

Summary

Test-only PR adding MTR coverage for mhnsw_ef_search boundary behavior.

Changes

• mysql-test/main/vector.test
• mysql-test/main/vector.result

Verification

cd build/mysql-test
./mariadb-test-run.pl --record main.vector

Pass (Storage Engine): aria, innodb, myisam.

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