fix(opensearch): keep the BM25 leg in hybrid search (#15760)

### What problem does this PR solve?

Fixes the OpenSearch side of #10747: hybrid search drops the keyword
(BM25) leg and
ends up doing plain vector search.

When a search has both a text and a vector leg, `OSConnection.search()`
throws the text
query away:

    del q["query"]
    q["query"] = {"knn": knn_query}

The text clause only stays on as a filter inside the knn query, so it
narrows the
candidate set but doesn't count towards scoring. So hybrid search on
OpenSearch behaves
like plain vector search, unlike the Elasticsearch backend.

What I changed:

- when both legs are present, send a real hybrid query
`{"hybrid": {"queries": [bm25, {"knn": ...}]}}` and let a
normalization-processor
  search pipeline score and combine the two legs
- only the actual filters (kb_id, available_int, ...) go in the knn
filter, not the
  text must clause
- create the pipeline on startup if it's missing, so there's no separate
provisioning
step. name and weights can be set under `os:` in service_conf.yaml, or
via
`OS_HYBRID_PIPELINE`; defaults are `ragflow_hybrid_pipeline` and `[0.5,
0.5]`
- normalization-processor needs OpenSearch 2.10+. on older clusters, or
when the
pipeline can't be created, log a warning and fall back to vector-only
instead of
  pointing at a pipeline that doesn't exist

This is only the hybrid-search fix; `create_doc_meta_idx` is already on
main.

Testing (there's no OpenSearch path in CI): added a unit test
(`test/unit_test/rag/utils/test_opensearch_hybrid_search.py`, no
services needed) that
checks the query built in each case — hybrid + pipeline param for
text+vector, plain knn
for vector-only, plain bool for text-only, the knn filter never carrying
the text
query_string, and the vector-only fallback when the pipeline isn't
available. Also ran
it against a real OpenSearch 2.19.1 container with a doc that matches
the keyword but
sits outside the knn top-k: pure knn returns `['D1','D2','D5']` (keyword
doc missing),
the hybrid query returns `['A','D1','D2','D5']` (keyword doc present).

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Signed-off-by: Danut Matei <matei.danut.dm@gmail.com>

conf/service_conf.yaml

@@ -27,6 +27,10 @@ os:
   hosts: 'http://localhost:1201'
   username: 'admin'
   password: 'infini_rag_flow_OS_01'
+  # Optional hybrid (BM25 + KNN) search tuning. The connector self-provisions the
+  # normalization search pipeline on start-up (requires OpenSearch >= 2.10).
+  # hybrid_search_pipeline: 'ragflow_hybrid_pipeline'
+  # hybrid_search_weights: [0.5, 0.5]   # [text/BM25 leg, vector/KNN leg]
 infinity:
   uri: 'localhost:23817'
   postgres_port: 5432