…urce files

Problem (Issue volcengine#1706):
When a directory contains only metadata files (.abstract / .overview) and
no actual resource files, the reindex executor incorrectly attempts to
process it, leading to errors or empty reindex operations.

Root cause:
_reindex_resource_vectors_from_entries in reindex_executor.py did not
check whether a directory has any reindexable resource files before
processing it. It would read the abstract/overview and proceed even when
no real resource content existed.

Fix:
- Added _directory_has_reindexable_resource_files() helper that checks
  whether a directory entry list contains any non-metadata resource files
- In _reindex_resource_vectors_from_entries, skip directories that have
  no reindexable resource files (metadata-only directories)
- Added comprehensive tests in test_admin_rebuild_api.py covering:
  - Pure metadata-only directory (should be skipped)
  - Directory with mixed metadata + resource files (should be processed)
  - Directory with only resource files (should be processed)

Files changed:
- openviking/service/reindex_executor.py  (+32 lines)
- tests/server/test_admin_rebuild_api.py   (+144 lines)

Verified: pytest tests/server/test_admin_rebuild_api.py passed (exit 0).