The gateway now sends the concurrency limit as a RateLimit-Policy
field from draft-ietf-httpapi-ratelimit-headers. The value is
"concurrency";q=<n>;qu="concurrent-requests". The invented
X-Concurrency-Limit header is gone.
The new concurrencyAutodetect option is off by default. When on,
the gateway learns a limit for each configured provider. A 429, 503,
504 or upstream timeout sets the limit to half the in-flight
requests, with a minimum of 1. Four consecutive successes add 1.
At 64 the limit counts as unknown again.
Responses served via a provider advertise the smaller of
maxConcurrency and the learned value. An explicit maxConcurrency
always wins. Routing, retry and failover do not change.
