`_env_expand_match` read `os.environ` directly, so under a multiplexed
gateway every secondary profile whose config.yaml carried
`${MATRIX_ACCESS_TOKEN}` (or `${env:...}`) expanded to the DEFAULT
profile's token loaded at startup -- each profile "had" the credential and
one inbound message fanned out across all of them. This is the residual
half of #84079 the secondary credential gate cannot see (the expanded
token is non-empty).
Add `_env_ref_lookup`: outside a secret scope it is the same
`os.environ.get`; inside a scope it goes through `get_secret`, which is
authoritative under multiplexing and an environ overlay otherwise -- the
same policy `gateway.config._getenv` and `get_env_value` already follow.
The cache env-snapshot (#58514) uses the same lookup so a scoped load is
not served another scope's cached expansion.
Teknium
b53c50cf8a