fix: fix alter collection failed for STRUCT sub-fields (#45041)

issue: https://github.com/milvus-io/milvus/issues/45001
ref: https://github.com/milvus-io/milvus/issues/42148

---------

Signed-off-by: SpadeA <tangchenjie1210@gmail.com>

internal/rootcoord/alter_collection_task.go

@@ -388,6 +388,17 @@ func ResetFieldProperties(coll *model.Collection, fieldName string, newProps []*
 			return nil
 		}
 	}
+	for _, structField := range coll.StructArrayFields {
+		if structField.Name == fieldName {
+			return merr.WrapErrParameterInvalidMsg("struct field has no properties to alter", fieldName)
+		}
+		for i, field := range structField.Fields {
+			if field.Name == fieldName {
+				structField.Fields[i].TypeParams = newProps
+				return nil
+			}
+		}
+	}
 	return merr.WrapErrParameterInvalidMsg("field %s does not exist in collection", fieldName)
 }
 
@@ -397,6 +408,16 @@ func GetFieldProperties(coll *model.Collection, fieldName string) ([]*commonpb.K
 			return field.TypeParams, nil
 		}
 	}
+	for _, structField := range coll.StructArrayFields {
+		if structField.Name == fieldName {
+			return nil, merr.WrapErrParameterInvalidMsg("struct field has no properties", fieldName)
+		}
+		for _, field := range structField.Fields {
+			if field.Name == fieldName {
+				return field.TypeParams, nil
+			}
+		}
+	}
 	return nil, merr.WrapErrParameterInvalidMsg("field %s does not exist in collection", fieldName)
 }