Add OceanBase coverage for embedding-failure text fallback

[wayyoungboy]

Follow-up from review of PR #1052.

Summary

Add OceanBase-focused coverage for search behavior when query embeddings cannot be generated and the search path must fall back to text retrieval.

Suggested coverage

• retrieval_mode=auto with no query embedding and a text query.
• retrieval_mode=fts, retrieval_mode=vector, and retrieval_mode=hybrid behavior when vectors are unavailable.
• Threshold and filter behavior for the fallback path.

Acceptance criteria

• The fallback behavior is covered by either a lightweight OceanBase-like unit test or a real backend integration test.
• Vector-only backends still return no false-positive results when vectors are unavailable.

[PsiACE]

PowerContext already falls back to full-text search when query embeddings are unavailable in auto mode. Explicit vector and hybrid requests return a capability error instead of incorrect results, so we are closing this issue as completed.

We are now developing PowerContext, the next generation of PowerMem, on the master branch.