Checks whether a forwarder token and environment ID are a usable
pair before a forwarder is installed with them.

Forwarder installers take both values from whoever is running the
install, and a typo in either one produces a forwarder that starts
cleanly and then silently never reports. Call this first and gate
the install on valid.

This is the one endpoint on this API that does NOT authenticate
with the Authorization header: the credential under test is
supplied in the request body instead, because the caller is trying
to find out whether it is a credential at all. Send nothing in the
Authorization header.

The response is deliberately a bare boolean. It never reveals
whether an environment exists, who owns it, or why a pair was
rejected.

Callers MUST treat HTTP 429 as "try again shortly", never as
"invalid". The endpoint is rate limited, and answering a 429 as
though it were a validation failure would block installs that are
perfectly well configured. Retry with backoff instead.

Only rejected answers count against the limit, so a large, correctly
configured rollout does not accumulate against it however many
endpoints it covers. Repeatedly asking about a pair that is wrong
is what will eventually be throttled.