Add OceanBase and SeekDB Go document engine (#17780)

## What changed

- add an OceanBase/SeekDB Go document engine using `database/sql` and
the existing MySQL driver
- preserve the Python connector's configuration, physical table names,
schema, index names, and ARRAY/JSON/VECTOR encodings
- implement chunk, memory, document metadata, skill, SQL, full-text,
vector, and fusion search paths
- support `DBMS_HYBRID_SEARCH.SEARCH` behind the existing feature flag,
with SQL fallback only when the package is unavailable
- wire the engine into retrieval, memory, metadata, vector hydration,
and SQL chat flows
- add Python/Go compatibility contracts, SQL mock tests, and an
integration-tagged round-trip test

---------

Co-authored-by: Jin Hai <haijin.chn@gmail.com>

internal/service/document/document_metadata.go

@@ -77,7 +77,7 @@ func (s *DocumentService) DeleteDocumentMetadata(ctx context.Context, docID stri
 	}
 
 	// Delete metadata using the document engine
-	err = s.docEngine.DeleteMetadataKeys(nil, docID, doc.KbID, keys, tenantID)
+	err = s.docEngine.DeleteMetadataKeys(ctx, docID, doc.KbID, keys, tenantID)
 	if err != nil {
 		return fmt.Errorf("failed to delete metadata: %w", err)
 	}
@@ -106,7 +106,7 @@ func (s *DocumentService) DeleteDocumentAllMetadata(ctx context.Context, docID s
 	}
 
 	// Delete entire document metadata
-	_, err = s.docEngine.DeleteMetadata(nil, condition, tenantID)
+	_, err = s.docEngine.DeleteMetadata(ctx, condition, tenantID)
 	if err != nil {
 		return fmt.Errorf("failed to delete document metadata: %w", err)
 	}