test(storage): add OceanBase embedding-failure text fallback coverage…

[kic635]

Summary

Fix #1094.
Add required-path unit coverage for OceanBase search when query embeddings are unavailable and the path must fall back to text retrieval (follow-up from PR #1052).
No production code changes.

Solution Description

Add tests/unit/test_oceanbase_embedding_fallback.py with two layers of coverage:

1. StorageAdapter routing
   • auto / fts / hybrid with query_embedding=None and a text query are forwarded to an OceanBase-like store
   • vector mode without embedding returns empty and does not call the store
   • vector-only OceanBase backends (hybrid_search=False) return no false-positive results
   • threshold / filter / sparse / fusion / candidate_limit controls are forwarded correctly
2. OceanBaseVectorStore production paths
   • real _vector_search safety belt: vectors=None / [] returns empty and does not call ann_search
   • FTS SQL generation and threshold behavior on the production FTS path
   • auto / hybrid without vectors still run FTS after the empty vector branch
   • weighted fusion with FTS-only results
   • remote parallel hybrid path
   • native hybrid falls back to application hybrid when vectors are missing
     Verified locally with:

python -m pytest tests/unit/test_oceanbase_embedding_fallback.py -v

[wayyoungboy]

LGTM