Fix: safe defaults for BaseModel dict fields (#24098)

Co-authored-by: Yongtao Huang <99629139+hyongtao-db@users.noreply.github.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

api/core/rag/models/document.py

@@ -2,7 +2,7 @@ from abc import ABC, abstractmethod
 from collections.abc import Sequence
 from typing import Any, Optional
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 
 class ChildDocument(BaseModel):
@@ -15,7 +15,7 @@ class ChildDocument(BaseModel):
     """Arbitrary metadata about the page content (e.g., source, relationships to other
         documents, etc.).
     """
-    metadata: dict = {}
+    metadata: dict = Field(default_factory=dict)
 
 
 class Document(BaseModel):
@@ -28,7 +28,7 @@ class Document(BaseModel):
     """Arbitrary metadata about the page content (e.g., source, relationships to other
         documents, etc.).
     """
-    metadata: dict = {}
+    metadata: dict = Field(default_factory=dict)
 
     provider: Optional[str] = "dify"