[bug] Uploads middleware drops human message metadata

[yetuge]

Before you start

• I searched existing issues and pull requests; this is not a duplicate.
• I can reproduce this on the latest main (827acf5).

Problem summary

UploadsMiddleware rebuilds the last HumanMessage from a subset of fields, silently dropping response_metadata and other message fields.

Affected area(s)

Agents / LangGraph — uploads middleware.

What happened?

UploadsMiddleware.before_agent() rewrites the last HumanMessage to prepend the <current_uploads> block. It constructs a fresh HumanMessage carrying only content, id, name, and additional_kwargs, so a message with response_metadata={"source": "gateway"} reaches the model with that field emptied.

This is the same partial-reconstruction family already fixed twice:

• ThreadDataMiddleware — fixed in fix(middleware): preserve human message metadata #5823 (issue [bug] ThreadDataMiddleware drops HumanMessage response metadata #5822)
• InputSanitizationMiddleware — both sites fixed in fix(middleware): preserve metadata during input sanitization #5875 (issue [bug] Input sanitization drops human message metadata #5874)
• PiiRedactionMiddleware._process_request — fix(middleware): preserve message fields when PII redaction rebuilds a user message #5968 (issue [bug] PII redaction drops human message metadata #5967)
• UploadsMiddleware.before_agent — still unfixed on main

The reproduction matches #5822 and #5874: a message carrying response_metadata passes through the middleware and comes out with an empty dict. Reachability is the same as those two — normalize_input() delegates to convert_to_messages, which preserves caller-supplied response_metadata on a role: human entry, so an API client can put the field there without going through the UI.

Minimal reproduction:

from langchain_core.messages import HumanMessage
from deerflow.agents.middlewares.uploads_middleware import UploadsMiddleware

mw = UploadsMiddleware(base_dir=str(tmp_path))
message = HumanMessage(
    content="check image",
    id="msg-1",
    additional_kwargs={"files": [{"filename": "img.png", "size": 3,
                                  "path": "/mnt/user-data/uploads/img.png",
                                  "status": "uploaded"}]},
    response_metadata={"source": "gateway"},
)
result = mw.before_agent({"messages": [message]}, runtime)
result["messages"][-1].content            # <current_uploads> block prepended — intended
result["messages"][-1].response_metadata  # {} — the field was dropped

Expected behavior

Prepending the uploads context should change only content and the deliberately updated additional_kwargs. Other fields — response_metadata, id, name, and the message subclass — should survive the rebuild, while the original request message stays unchanged and unaliased.

Steps to reproduce

1. Build a HumanMessage whose additional_kwargs.files lists an uploaded file, and which carries response_metadata={"source": "gateway"}.
2. Call UploadsMiddleware.before_agent(state, runtime) with it as the last message.
3. Inspect the returned message: response_metadata is {} instead of {"source": "gateway"}.

Additional context

I have a fix plus a red test ready (backend/tests/test_uploads_middleware_core_logic.py, verified red on main and green on the branch); opening the PR next.