feat(agents): aggregate_metrics proxy helper on SimpleResponsesAPIAgent #2250

Problem

Eight agent servers override aggregate_metrics with the same ~8 line proxy to their resources server: speed_bench_agent, non_executing_simple_agent, browsecomp_agent, gymnasium_agent, finance_agent, stirrup_agent, simple_agent, remote_agent. remote_agent additionally bounds the hop with asyncio.wait_for(600s) because the ServerClient call otherwise retries connection errors forever (see #2248); a dead resources server at end of run would hang the collector after all rollouts are on disk.

Proposal

An additive helper on SimpleResponsesAPIAgent:

async def proxy_aggregate_metrics(self, server_name, body, timeout_secs=600.0) -> AggregateMetrics

POST /aggregate_metrics to the named server, raise_for_status, validate, bounded by wait_for when timeout_secs is set. Opt-in per agent; the default local aggregation is untouched (harbor_agent deliberately keeps local aggregation despite having a resources server).

Origin: #2163 (comment)