community: Fix all page numbers were the same for _BaseGoogleVertexAISearchRetriever (#19175)

- Description:
- This pull request is to fix a bug where page numbers were not set
correctly. In the current code, all chunks share the same metadata
object doc_metadata, so the page number is set with the same value for
all documents. To fix this, I changed to using separate metadata objects
for each chunk.
- Issue:
  - None
- Dependencies:
  - No additional dependencies are required for this change.
- Twitter handle:
  - @eycjur

- Test
- Even if it's not a bug, there are cases where everything ends up with
the same number of pages, so it's very difficult for me to write
integration tests.

libs/community/langchain_community/retrievers/google_vertex_ai_search.py

@@ -137,14 +137,15 @@ class _BaseGoogleVertexAISearchRetriever(BaseModel):
                 continue
 
             for chunk in derived_struct_data[chunk_type]:
-                doc_metadata["source"] = derived_struct_data.get("link", "")
+                chunk_metadata = doc_metadata.copy()
+                chunk_metadata["source"] = derived_struct_data.get("link", "")
 
                 if chunk_type == "extractive_answers":
-                    doc_metadata["source"] += f":{chunk.get('pageNumber', '')}"
+                    chunk_metadata["source"] += f":{chunk.get('pageNumber', '')}"
 
                 documents.append(
                     Document(
-                        page_content=chunk.get("content", ""), metadata=doc_metadata
+                        page_content=chunk.get("content", ""), metadata=chunk_metadata
                     )
                 )