The lease, stall and probe-TTL guards only ran inside GatewaySettings.from_env,
so from_settings() and direct construction could produce settings that violate
the class's own invariants: the permit lease could expire mid-request (silently
exceeding the concurrency quota), a normal slow first token could be killed as a
stall, and a half-open probe could be taken over while still in flight.
Guards move into __post_init__ as _validate_* methods, matching every frozen
dataclass in types.py, so all six factories plus dataclasses.replace are covered
by one check. Adds a non-empty sources invariant that previously only from_env
enforced. Messages now name fields instead of env keys, since callers who build
settings by hand never set those keys.
iomgaa
b8f738f8cb