feat(threads): history endpoint reads messages from event store

- POST /api/threads/{thread_id}/history now combines two data sources:
  checkpointer for checkpoint_id, metadata, title, thread_data;
  event store for messages (complete history, not truncated by summarization)
- Strip internal LangGraph metadata keys from response
- Remove full channel_values serialization in favor of selective fields

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

backend/app/gateway/routers/threads.py

@@ -561,13 +561,36 @@ async def update_thread_state(thread_id: str, body: ThreadStateUpdateRequest, re
 
 @router.post("/{thread_id}/history", response_model=list[HistoryEntry])
 async def get_thread_history(thread_id: str, body: ThreadHistoryRequest, request: Request) -> list[HistoryEntry]:
-    """Get checkpoint history for a thread."""
+    """Get checkpoint history for a thread.
+
+    Combines data from two sources:
+    - **Checkpointer**: checkpoint_id, parent_checkpoint_id, metadata,
+      values.title, values.thread_data
+    - **RunEventStore**: values.messages (complete conversation history,
+      not affected by summarization truncation)
+    """
+    from app.gateway.deps import get_run_event_store
+
     checkpointer = get_checkpointer(request)
 
     config: dict[str, Any] = {"configurable": {"thread_id": thread_id}}
     if body.before:
         config["configurable"]["checkpoint_id"] = body.before
 
+    # Fetch messages from event store (full history, not truncated by summarization)
+    event_store = get_run_event_store(request)
+    try:
+        all_messages = await event_store.list_messages(thread_id, limit=10_000)
+    except Exception:
+        logger.warning("Failed to load messages from event store for thread %s", thread_id, exc_info=True)
+        all_messages = []
+
+    # Group messages by run_id for per-checkpoint assembly
+    messages_by_run: dict[str, list[dict]] = {}
+    for msg in all_messages:
+        run_id = msg.get("run_id", "")
+        messages_by_run.setdefault(run_id, []).append(msg.get("content", {}))
+
     entries: list[HistoryEntry] = []
     try:
         async for checkpoint_tuple in checkpointer.alist(config, limit=body.limit):
@@ -583,16 +606,33 @@ async def get_thread_history(thread_id: str, body: ThreadHistoryRequest, request
 
             channel_values = checkpoint.get("channel_values", {})
 
+            # Build values: title + thread_data from checkpoint, messages from event store
+            values: dict[str, Any] = {}
+            if title := channel_values.get("title"):
+                values["title"] = title
+            if thread_data := channel_values.get("thread_data"):
+                values["thread_data"] = thread_data
+
+            # Attach all messages from event store (not just this checkpoint's run)
+            if all_messages:
+                values["messages"] = [m.get("content", {}) for m in all_messages]
+
             # Derive next tasks
             tasks_raw = getattr(checkpoint_tuple, "tasks", []) or []
             next_tasks = [t.name for t in tasks_raw if hasattr(t, "name")]
 
+            # Strip LangGraph internal keys from metadata
+            user_meta = {k: v for k, v in metadata.items() if k not in ("created_at", "updated_at", "step", "source", "writes", "parents")}
+            # Keep step for ordering context
+            if "step" in metadata:
+                user_meta["step"] = metadata["step"]
+
             entries.append(
                 HistoryEntry(
                     checkpoint_id=checkpoint_id,
                     parent_checkpoint_id=parent_id,
-                    metadata=metadata,
+                    metadata=user_meta,
-                    values=serialize_channel_values(channel_values),
+                    values=values,
                     created_at=str(metadata.get("created_at", "")),
                     next=next_tasks,
                 )