Say what a node returned when it is not a dict of ports

Outputs are keyed by port, so a bare value cannot be one. The single mapping
point every caller routes through raised a bare AttributeError from
retval.items(); it now names the problem, and the message reaches the node the
way its other errors do.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H7LwYgJfpkbLCTeiAf8U4A

backend/app/flow/nodes/base.py

@@ -28,6 +28,10 @@ logger = logging.getLogger(__name__)
 NodeResult: TypeAlias = "StateBackend | dict[str, Any] | None"
 
 
+class NodeOutputError(TypeError):
+    """A node function returned something that cannot be mapped onto ports."""
+
+
 class Node:
     """
     A pipeline node that wraps a function with typed inputs/outputs.
@@ -251,10 +255,15 @@ class Node:
             kwargs[spec.port] = value
         return kwargs
 
-    def _to_messages(self, retval: dict[str, Any] | None) -> dict[str, Any] | None:
+    def _to_messages(self, retval: Any) -> dict[str, Any] | None:
         """Map a function's port-keyed return value onto message names."""
         if not retval:
             return None
+        if not isinstance(retval, dict):
+            raise NodeOutputError(
+                f"'{self.local_id}' returned {type(retval).__name__}. Outputs are "
+                "keyed by port, so return a dict like {'out': value}, or None."
+            )
         by_port = {s.port: s for s in self.output_ports if s.name}
         outputs = {}
         for key, value in retval.items():