Fix: add missing "mom" field to infinity_mapping.json for parent-child chunker (#13821)

### What problem does this PR solve?

When using Infinity as DOC_ENGINE with parent-child chunker enabled,
vector insertion fails because the "mom" field is missing from the index
mapping. This fix adds the required field to resolve the issue.

### Type of change

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

Author: asiroliu

conf/infinity_mapping.json

@@ -3,6 +3,7 @@
 	"doc_id": {"type": "varchar", "default": ""},
 	"kb_id": {"type": "varchar", "default": "", "index_type": {"type": "secondary", "cardinality": "low"}},
 	"mom_id": {"type": "varchar", "default": ""},
+	"mom": {"type": "varchar", "default": ""},
 	"create_time": {"type": "varchar", "default": ""},
 	"create_timestamp_flt": {"type": "float", "default": 0.0},
 	"img_id": {"type": "varchar", "default": ""},