fix(delegate): inherit parent fallback_chain in _build_child_agent

_build_child_agent constructed child AIAgents without passing
fallback_model, leaving _fallback_chain=[] for every subagent.
When a subagent hit a rate-limit or credential exhaustion the
runtime fallback check (run_agent.py:7486 / 12267) found an empty
chain and failed immediately — even though the parent agent was
configured with fallback_providers and would have recovered.

The cron scheduler already propagates fallback_model correctly
(scheduler.py:1038). Fix closes the parity gap by reading the
parent's _fallback_chain (the normalised list form accepted by
AIAgent's fallback_model parameter) and threading it through.

Empty chains coerce to None so AIAgent initialises _fallback_chain=[]
as usual rather than iterating an empty list.

tools/delegate_tool.py

@@ -1026,6 +1026,12 @@ def _build_child_agent(
     except Exception as exc:
         logger.debug("Could not load delegation reasoning_effort: %s", exc)
 
+    # Inherit the parent's fallback provider chain so subagents can recover
+    # from rate-limits and credential exhaustion exactly like the top-level
+    # agent does.  _fallback_chain is a list accepted by AIAgent's
+    # fallback_model parameter (which handles both list and dict forms).
+    parent_fallback = getattr(parent_agent, "_fallback_chain", None) or None
+
     child = AIAgent(
         base_url=effective_base_url,
         api_key=effective_api_key,
@@ -1038,6 +1044,7 @@ def _build_child_agent(
         max_tokens=getattr(parent_agent, "max_tokens", None),
         reasoning_config=child_reasoning,
         prefill_messages=getattr(parent_agent, "prefill_messages", None),
+        fallback_model=parent_fallback,
         enabled_toolsets=child_toolsets,
         quiet_mode=True,
         ephemeral_system_prompt=child_prompt,