Fix hybrid search bindings, result limit and SQLAlchemy row mappings

[rudycelekli]

Agent information

Existing agent: RAG Pipeline Engineer
File: engineering/engineering-rag-pipeline-engineer.md

Motivation and change

The hybrid_search example parses :embedding::vector as a truncated SQLAlchemy bind, doubles the final result limit instead of just each candidate pool, and calls dict() on SQLAlchemy Row objects. Use explicit CAST bindings, separate candidate_k from top_k, and materialize mapping rows. These changes repair one documented query flow.

Testing

7 native SQLAlchemy 2 tests pass; unchanged source fails 3. Tests compile the actual text query through the PostgreSQL dialect, inspect binds and both candidate/final limits, and map real SQLite Row objects through a controlled async adapter. Empty-result, metadata-filter and RRF expression controls pass.

Changed-agent lint passes and full 282-agent conversion across 15 tools passes all 32 checks on this signed head. Content checksum drift remains advisory; no generated outputs or manifest changes are included.

Scope and limitations

No live PostgreSQL/pgvector service or retrieval-quality benchmark was run. Compiler and native Row tests establish these specific interface failures; vector codec/database configuration remains external to this example.

Only one existing agent file changes, retaining personality, structure and unrelated examples. Candidate listed in issue #917 before publication. Investigation and preparation were AI-assisted; native evidence was executed and exact source independently reviewed.

References: SQLAlchemy text binds, Result.mappings.