fix: close file handles when loading JSON mapping in doc store connectors (#12904)

**What problem does this PR solve?**

When loading JSON mapping/schema files, the code used
json.load(open(path)) without closing the file. The file handle stayed
open until garbage collection, which can leak file descriptors under
load (e.g. repeated reconnects or migrations).

**Type of change**
[x] Bug Fix (non-breaking change which fixes an issue)
**Change**
Replaced json.load(open(...)) with a context manager so the file is
closed after loading:
with open(fp_mapping, "r") as f:    ... = json.load(f)

**Files updated**
rag/utils/opensearch_conn.py – mapping load (1 place)
common/doc_store/es_conn_base.py – mapping load + doc_meta_mapping load
(2 places)
common/doc_store/infinity_conn_base.py – schema loads in _migrate_db,
doc metadata table creation, and SQL field mapping (4 places)
Behavior is unchanged; only resource handling is fixed.

Co-authored-by: Gittensor Miner <miner@gittensor.io>

rag/utils/opensearch_conn.py

@@ -72,7 +72,8 @@ class OSConnection(DocStoreConnection):
             msg = f"OpenSearch mapping file not found at {fp_mapping}"
             logger.error(msg)
             raise Exception(msg)
-        self.mapping = json.load(open(fp_mapping, "r"))
+        with open(fp_mapping, "r") as f:
+            self.mapping = json.load(f)
         logger.info(f"OpenSearch {settings.OS['hosts']} is healthy.")
 
     """