fix(delegate): move _saved_tool_names assignment to correct scope

The merge at e7844e9c re-introduced a line in _build_child_agent() that
references _saved_tool_names — a variable only defined in _run_single_child().
This caused NameError on every delegate_task call, completely breaking
subagent delegation.

Moves the child._delegate_saved_tool_names assignment to _run_single_child()
where _saved_tool_names is actually defined, keeping the save/restore in the
same scope as the try/finally block.

Adds two regression tests from PR #2038 (YanSte).
Also fixes the same issue reported in PR #2048 (Gutslabs).

Co-authored-by: Yannick Stephan <yannick.stephan@gmail.com>
Co-authored-by: Guts <gutslabs@users.noreply.github.com>

tools/delegate_tool.py

@@ -232,8 +232,6 @@ def _build_child_agent(
         tool_progress_callback=child_progress_cb,
         iteration_budget=shared_budget,
     )
-    child._delegate_saved_tool_names = list(_saved_tool_names)
-
     # Set delegation depth so children can't spawn grandchildren
     child._delegate_depth = getattr(parent_agent, '_delegate_depth', 0) + 1
 
@@ -270,6 +268,7 @@ def _run_single_child(
     # save/restore happens in the same scope as the try/finally.
     import model_tools
     _saved_tool_names = list(model_tools._last_resolved_tool_names)
+    child._delegate_saved_tool_names = _saved_tool_names
 
     try:
         result = child.run_conversation(user_message=goal)