Fix retrieval function when metadata_condtion is specified in retrieval API (#13473)

### What problem does this PR solve?

Fix https://github.com/infiniflow/ragflow/issues/13388

The following command returns empty when there is doc with the meta data
```
curl --request POST \
     --url http://localhost:9222/api/v1/retrieval \
     --header 'Content-Type: application/json' \
     --header 'Authorization: Bearer ragflow-fO3mPFePfLgUYg8-9gjBVVXbvHqrvMPLGaW0P86PvAk' \
     --data '{
          "question": "any question",
          "dataset_ids": ["9bb4f0591b8811f18a4a84ba59049aa3"],
           "metadata_condition": {
            "logic": "and",
            "conditions": [
              {
                "name": "character",
                "comparison_operator": "is",
                "value": "刘备"
              }
            ]
          }
     }'
```

When metadata_condtion is specified in the retrieval API, it is
converted to doc_ids and doc_ids is passed to retrieval function.
In retrieval funciton, when doc_ids is explicitly provided , we should
bypass threshold.


### Type of change

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

api/apps/sdk/doc.py

@@ -1682,7 +1682,7 @@ async def retrieval_test(tenant_id):
     if not doc_ids:
         metadata_condition = req.get("metadata_condition")
         if metadata_condition:
-            metas = DocMetadataService.get_meta_by_kbs(kb_ids)
+            metas = DocMetadataService.get_flatted_meta_by_kbs(kb_ids)
             doc_ids = meta_filter(metas, convert_conditions(metadata_condition), metadata_condition.get("logic", "and"))
             # If metadata_condition has conditions but no docs match, return empty result
             if not doc_ids and metadata_condition.get("conditions"):