The changed-tests workflow overrode the suite's `--reruns 1` with `--reruns 0`, so a
transport blip failed a pass that pytest.ini already scopes to network errors and
5xx responses. Pass 2 of run 33692484803 also went red 15s after a model write with
"no healthy deployments": the barrier only polled /v1/models through nginx, which
proves one gateway converged, and the next request rolled the other. The stack now
exports LITELLM_PROXY_REPLICA_URLS, the barrier polls every replica with the full
budget before settling, and up.sh refuses to boot without DD_API_KEY, since the
gateway config enables the datadog callback on every run
Adds a required-check candidate that selects the tests/e2e test files a PR added or
modified, boots a stage-mirror stack on the runner (migrations, backend, two gateway
processes behind nginx, Postgres, Jaeger, TLS cluster Valkey), and runs those files
three times with retries off. The run job sits behind the e2e-changed GitHub
environment, so a reviewer approves each run before the OIDC token that reads the
provider keys from AWS Secrets Manager exists. Supersedes #34981
