fix(journal): unwrap Command tool results in on_tool_end

Tools that update graph state (e.g. ``present_files``) return
``Command(update={'messages': [ToolMessage(...)], 'artifacts': [...]})``.
LangGraph later unwraps the inner ``ToolMessage`` into checkpoint state,
but ``RunJournal.on_tool_end`` was receiving the ``Command`` object
directly via the LangChain callback chain and storing
``str(Command(update={...}))`` as the tool_result content.

This produced a visible divergence between the event-store and the
checkpoint for any thread that used a Command-returning tool, blocking
the event-store-backed history fix in the follow-up commit. Concrete
example from thread ``6d30913e-dcd4-41c8-8941-f66c716cf359`` (seq=48):
checkpoint had ``'Successfully presented files'`` while event_store
stored the full Command repr.

The fix detects ``Command`` in ``on_tool_end``, extracts the first
``ToolMessage`` from ``update['messages']``, and lets the existing
ToolMessage branch handle the ``model_dump()`` path. Legacy rows still
containing the Command repr are separately cleaned up by the history
helper in the follow-up commit.

Tests:
- ``test_tool_end_unwraps_command_with_inner_tool_message`` — unit test
  of the unwrap branch with a constructed Command
- ``test_tool_invoke_end_to_end_unwraps_command`` — end-to-end via
  ``CallbackManager`` + ``tool.invoke`` to exercise the real LangChain
  dispatch path that production uses, matching the repro shape from
  ``present_files``
- Counter-proof: temporarily reverted the patch, both tests failed with
  the exact ``Command(update={...})`` repr that was stored in the
  production SQLite row at seq=48, confirming LangChain does pass the
  ``Command`` through callbacks (the unwrap is load-bearing)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/packages/harness/deerflow/runtime/journal.py

@@ -246,6 +246,19 @@ class RunJournal(BaseCallbackHandler):
 
     def on_tool_end(self, output: Any, *, run_id: UUID, **kwargs: Any) -> None:
         from langchain_core.messages import ToolMessage
+        from langgraph.types import Command
+
+        # Tools that update graph state return a ``Command`` (e.g.
+        # ``present_files``). LangGraph later unwraps the inner ToolMessage
+        # into checkpoint state, so to stay checkpoint-aligned we must
+        # extract it here rather than storing ``str(Command(...))``.
+        if isinstance(output, Command):
+            update = getattr(output, "update", None) or {}
+            inner_msgs = update.get("messages") if isinstance(update, dict) else None
+            if isinstance(inner_msgs, list):
+                inner_tool_msg = next((m for m in inner_msgs if isinstance(m, ToolMessage)), None)
+                if inner_tool_msg is not None:
+                    output = inner_tool_msg
 
         # Extract fields from ToolMessage object when LangChain provides one.
         # LangChain's _format_output wraps tool results into a ToolMessage