ThreadingHTTPServer request threads do not inherit the gateway's profile
ContextVars, so security.authenticate()/is_trusted_peer() read the
process-global A2A_* env on every request — every secondary profile's
listener authenticated against the default profile's tokens. Capture an
immutable A2ASecurityContext at adapter construction (which runs inside
_profile_runtime_scope for secondary profiles) and have the request
handler consult it instead of re-reading env per request.
Salvage note: the original `get_secret() / except UnscopedSecretError:
os.getenv()` fallthrough in _startup_env was replaced by _profile_scoped()
gating (Buzz/Raft pattern) — inside a secondary profile's scope the scope
is authoritative and a miss never falls through to os.environ.
Salvaged from #80956.
8167cfa4b6