fix(delegate): remove model-facing max_iterations override; config is authoritative (#14732)

Previously delegate_task exposed 'max_iterations' in its JSON schema and used
`max_iterations or default_max_iter` — so a model guessing conservatively (or
copy-pasting a docstring hint like 'Only set lower for simple tasks') could
silently shrink a subagent's budget below the user's configured
delegation.max_iterations. One such call this session capped a deep forensic
audit at 40 iterations while the user's config was set to 250.

Changes:
- Drop 'max_iterations' from DELEGATE_TASK_SCHEMA['parameters']['properties'].
  Models can no longer emit it.
- In delegate_task(): ignore any caller-supplied max_iterations, always use
  delegation.max_iterations from config. Log at debug if a stale schema or
  internal caller still passes one through.
- Keep the Python kwarg on the function signature for internal callers
  (_build_child_agent tests pass it through the plumbing layer).
- Update test_schema_valid to assert the param is now absent (intentional
  contract change, not a change-detector).

tests/tools/test_delegate.py

@@ -69,7 +69,10 @@ class TestDelegateRequirements(unittest.TestCase):
         self.assertIn("tasks", props)
         self.assertIn("context", props)
         self.assertIn("toolsets", props)
-        self.assertIn("max_iterations", props)
+        # max_iterations is intentionally NOT exposed to the model — it's
+        # config-authoritative via delegation.max_iterations so users get
+        # predictable budgets.
+        self.assertNotIn("max_iterations", props)
         self.assertNotIn("maxItems", props["tasks"])  # removed — limit is now runtime-configurable