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).

tools/delegate_tool.py

@@ -1558,7 +1558,18 @@ def delegate_task(
     # Load config
     cfg = _load_config()
     default_max_iter = cfg.get("max_iterations", DEFAULT_MAX_ITERATIONS)
-    effective_max_iter = max_iterations or default_max_iter
+    # Model-supplied max_iterations is ignored — the config value is authoritative
+    # so users get predictable budgets. The kwarg is retained for internal callers
+    # and tests; a model-emitted value here would only shrink the budget and
+    # surprise the user mid-run. Log and drop it if one slips through from a
+    # cached tool schema or a stale provider.
+    if max_iterations is not None and max_iterations != default_max_iter:
+        logger.debug(
+            "delegate_task: ignoring caller-supplied max_iterations=%s; "
+            "using delegation.max_iterations=%s from config",
+            max_iterations, default_max_iter,
+        )
+    effective_max_iter = default_max_iter
 
     # Resolve delegation credentials (provider:model pair).
     # When delegation.provider is configured, this resolves the full credential
@@ -2098,13 +2109,6 @@ DELEGATE_TASK_SCHEMA = {
                     "When provided, top-level goal/context/toolsets are ignored."
                 ),
             },
-            "max_iterations": {
-                "type": "integer",
-                "description": (
-                    "Max tool-calling turns per subagent (default: 50). "
-                    "Only set lower for simple tasks."
-                ),
-            },
             "role": {
                 "type": "string",
                 "enum": ["leaf", "orchestrator"],