fix(opensearch): repair document-metadata path broken by #14577 (#15393)

### What problem does this PR solve?

Document metadata is completely broken on the OpenSearch backend
(`DOC_ENGINE=opensearch`). Both failures were introduced by #14577,
which added
a doc-metadata dispatch surface but only validated it against
Elasticsearch.

**1. Index creation rejected (`mapper_parsing_exception`).**
`OSConnection.create_doc_meta_idx` feeds `conf/doc_meta_es_mapping.json`
verbatim to OpenSearch. That file declares a top-level `"dynamic":
"runtime"`.
Runtime fields are Elasticsearch-only; OpenSearch cannot parse the
value:

mapper_parsing_exception: Could not convert [dynamic.dynamic] to boolean
(400)

**2. `search()` signature mismatch (`TypeError`).**
`DocMetadataService` (added by #14577) calls `docStoreConn.search(...)`
with
snake_case kwargs (`select_fields=`, `index_names=`,
`knowledgebase_ids=`, …),
matching `ESConnection.search`. But `OSConnection.search` still uses
camelCase
parameters (`selectFields`, `indexNames`, `knowledgebaseIds`, …):

TypeError: OSConnection.search() got an unexpected keyword argument
'select_fields'

The UI then shows "0 fields" for every document on OpenSearch.

### Fix

1. In `OSConnection.create_doc_meta_idx`, normalize a top-level
`"dynamic": "runtime"` to `True` **for the OpenSearch request only**.
The
shared mapping file is left untouched, so the Elasticsearch backend
keeps its
runtime-field behavior. Dynamic field discovery is preserved on
OpenSearch.
2. Rename the `OSConnection.search()` parameters (and their in-method
local
uses) from camelCase to snake_case so they match `ESConnection.search()`
and
the `DocMetadataService` call sites. The change is confined to
`search()`;
`get/insert/update/delete` keep their existing positional signatures
(they
   are called positionally from `rag/nlp/search.py`).

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

### Affected backends
OpenSearch only. Elasticsearch, Infinity and OceanBase are untouched.

### How to reproduce
1. `DOC_ENGINE=opensearch`, restart the stack.
2. Upload/parse a document, then open the dataset's document list / set
metadata.
- Before: index creation 400s (`Could not convert [dynamic.dynamic]`),
and/or
     `TypeError ... 'select_fields'`; document metadata shows 0 fields.

### Risk & backward compatibility
- ES default deployment: no change. `doc_meta_es_mapping.json` is not
modified,
  so ES still receives `"dynamic": "runtime"`.
- `search()` rename is internal; the only kwarg caller
(`DocMetadataService`)
  already uses the snake_case names this PR aligns to.

### Test plan
- [ ] `DOC_ENGINE=opensearch`: per-tenant `ragflow_doc_meta_*` index is
created
(no `mapper_parsing_exception`); document metadata reads/writes work.
- [ ] `DOC_ENGINE=elasticsearch` regression: doc-meta index still
created with
      runtime mapping; metadata unchanged.