Reshape of the salvaged fix from #114513 (@whyyagswhy):
- ``CredentialPool.reclaim(credential_id, model=)`` is the pool-owned answer to "is the
benched entry back?": it runs under the pool lock, clears the elapsed cooldown and
refreshes the token exactly as ``select()`` would, but never bumps ``request_count``
or round-robin order. The contributor's version called the private
``_available_entries()`` outside the lock (its docstring requires the lock: it prunes
and persists) and left the entry marked ``exhausted`` in the pool after the swap.
- ``_rotate_and_swap`` arms the revert only when nothing is armed yet, so a chained
429 (preferred → fallback → third) still returns to the PREFERRED entry rather than
the middle one.
- A deliberate ``/model`` switch (``_finish_switch``) cancels the pending revert with
the rest of the fallback state; dropped the ``_credential_pool_rotated_to`` "moved
by hand" heuristic and the ``_provider_fallback_active`` guard (unreachable: the
hook only runs on the ``not _fallback_activated`` branch).
- Tests trimmed to two invariants on a real ``CredentialPool`` through the real
``recover_with_credential_pool`` → ``restore_primary_runtime`` path: (1) stays on the
fallback while the bench holds, moves back once it lifts, ``_fallback_activated``
and the model untouched; (2) a 401 bench does not arm a revert.
- Docs: credential-pools "Error Recovery" describes the switch-back.
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