rag: no test drives the legacy RAGBackend::search route this PR fixes #1139

Follow-up from #1137 by @Adityakk9031 — thanks again for that PR!

What

#1137 threads config_.bm25_k1 / bm25_b into BM25Index::search (core/src/features/rag/rag_backend.cpp:415) and config_.rrf_k into fuse_results (:418), but nothing in the tree executes that route: its only caller, core/tests/rag_backend_thread_safety_test.cpp:52 (backend.search("hello", 1)), is excluded from the build (core/tests/CMakeLists.txt:1588-1591 — it targets the removed IEmbeddingProvider / set_embedding_provider API), and every test the PR adds drives the primitive layers instead — core/tests/test_rag_fusion.cpp:76-104 calls BM25Index::search and fuse_rankings directly, and core/tests/test_advanced_modality_proto_abi.cpp:1985 onward goes through rac_rag_search_proto → retrieve → run_rag_retrieve. Reverting the three headline lines would leave every test green.

Why it matters

Issue #1128's second acceptance box — "core/tests/rag_backend_thread_safety_test.cpp still passes against whichever shape lands" — cannot be checked by anyone, CI included, and the silent divergence this work removed can return unnoticed the next time someone wires a caller to RAGBackend::search.

Suggested approach

Either add a focused test that constructs a RAGBackend with a non-default RAGBackendConfig and asserts the fused scores move with bm25_k1 / bm25_b / rrf_k (the mock ops at core/tests/test_advanced_modality_proto_abi.cpp:582 and :404 are reusable), or rewrite rag_backend_thread_safety_test.cpp against the current constructor and re-add it to core/tests/CMakeLists.txt. Deleting the legacy route instead and repointing its caller at retrieve would also close it, and is what #1128's alternative option contemplated.

Done when

• Some test fails if the legacy route stops honouring bm25_k1 / bm25_b / rrf_k, or the route is deleted.
• rag_backend_thread_safety_test.cpp is either built and passing or removed with its exclusion note.

Not blocking #1137. @Adityakk9031, you know this code well now — you're welcome to take this one if you're interested.

Opened with help from Claude Code and Codex.

Reviewed with help from Claude Code and Codex.