Heroku's connection-status probe used a static ProbeURL, which the
generic probe issues with `Accept: application/json`. Heroku negotiates
the API version through the Accept media type and returns 400 for an
unversioned request, which doProbeRequest reads as "connected" -- so the
probe never caught a revoked token (it only surfaced at the first
ListAccounts).
Probe via a probeHeroku closure that sends
`Accept: application/vnd.heroku+json; version=3` instead. Verified live:
a dead token returns 400 with application/json but 401 with the
versioned header, which doProbeRequest correctly maps to rejected.
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>
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