fix(history): strip base64 image data from REST endpoint responses (#3535)

ViewImageMiddleware persists full base64 image payloads in hide_from_ui
human messages inside checkpoints. All REST endpoints that returned
serialize_channel_values(channel_values) sent these multi-megabyte
payloads to the frontend, freezing the UI on threads with images.

Add strip_data_url_image_blocks() to remove data:-scheme image_url
content blocks from hide_from_ui messages, and
serialize_channel_values_for_api() as a convenience wrapper used by all
six affected call sites across threads, runs, and thread_runs routers.
SSE streaming is unaffected (still uses serialize_channel_values).

Fixes #3496

backend/app/gateway/routers/runs.py

@@ -18,7 +18,7 @@ from app.gateway.deps import get_checkpointer, get_feedback_repo, get_run_event_
 from app.gateway.pagination import trim_run_message_page
 from app.gateway.routers.thread_runs import RunCreateRequest
 from app.gateway.services import sse_consumer, start_run, wait_for_run_completion
-from deerflow.runtime import serialize_channel_values
+from deerflow.runtime import serialize_channel_values_for_api
 
 logger = logging.getLogger(__name__)
 router = APIRouter(prefix="/api/runs", tags=["runs"])
@@ -82,7 +82,7 @@ async def stateless_wait(body: RunCreateRequest, request: Request) -> dict:
             if checkpoint_tuple is not None:
                 checkpoint = getattr(checkpoint_tuple, "checkpoint", {}) or {}
                 channel_values = checkpoint.get("channel_values", {})
-                return serialize_channel_values(channel_values)
+                return serialize_channel_values_for_api(channel_values)
         except Exception:
             logger.exception("Failed to fetch final state for run %s", record.run_id)