Stripped the resource_exhausted status token before classifying the
remaining provider message. Bare or opaque status errors still use the
short model-capacity backoff, while explicit quota, rate-limit, capacity,
or server details remain authoritative.
Added regression coverage for a Connect resource_exhausted trailer with
an explicit quota-exceeded body.
Fixes#7032
(cherry picked from commit ee04c065692e34ccabe1a07b45b5f5987297ff84)
