Add checkConnectionCapacity guard with 429 + Retry-After in handleChat().
Introduce OMNI_MAX_CONCURRENT_CONNECTIONS env-bound cap, disabled (0) by
default so existing deployments are unaffected until an operator opts in.
Reconstructed from PR #6590, isolating only the backpressure change —
the original branch also carried unrelated headroom/docker/perf work
from the author's separate #6572 branch.
Co-authored-by: oyi77 <oyi77@users.noreply.github.com>
Co-authored-by: diegosouzapw <8016841+diegosouzapw@users.noreply.github.com>
