fix: report subagent status as completed when summary exists (#3829)

When a subagent hit max_iterations, status was always 'failed' even
if it produced a usable summary via _handle_max_iterations(). This
happened because the status check required both completed=True AND
a summary, but completed is False whenever max_iterations is reached
(run_agent.py line 7969).

Now gates status on whether a summary was produced — if the subagent
returned a final_response, the parent has usable output regardless of
iteration budget. The exit_reason field already distinguishes
'completed' vs 'max_iterations' for anything that needs to know how
the task ended.

Closes #1899.

tools/delegate_tool.py

@@ -289,7 +289,10 @@ def _run_single_child(
 
         if interrupted:
             status = "interrupted"
-        elif completed and summary:
+        elif summary:
+            # A summary means the subagent produced usable output.
+            # exit_reason ("completed" vs "max_iterations") already
+            # tells the parent *how* the task ended.
             status = "completed"
         else:
             status = "failed"