fix(credential_pool): recover exhausted entries on endpoint changes

[DanielLSM]

Symptom

A credential pool entry can remain exhausted on a stale base_url even after runtime provider resolution has detected a different working endpoint.

The concrete case is Z.AI regular vs coding-plan API surfaces: endpoint detection can cache the working coding-plan endpoint in provider_state.zai.detected_endpoint, while the pool entry remains stuck on the regular endpoint and is skipped before that newer auth state is used.

Root cause

CredentialPool._available_entries() checks exhaustion before syncing endpoint state from auth.json. For providers with multiple API surfaces, that leaves the pool entry stranded on an endpoint that no longer reflects the provider's detected working URL.

Fix

• Add _sync_endpoint_from_provider_state() to adopt provider_state.<provider>.detected_endpoint when it differs from the pool entry's cached base_url.
• Clear exhaustion only when the detected endpoint actually changes, so the entry can be retried on the working endpoint immediately.
• Re-probe provider endpoint detectors after an exhausted entry's cooldown expires, before returning the entry to callers.
• Keep the entry exhausted if no endpoint currently works, preserving the original error context.

The sync/re-probe mechanism is provider-agnostic; Z.AI is the first opted-in detector via _PROVIDER_ENDPOINT_DETECTORS.

Related context

• Related to Transient 401 blacklists credential for 24 hours — breaks single-key setups #5668's credential-pool cooldown/recovery path, but with a different root cause: stale provider endpoint state rather than transient auth failure.
• Similar class of stale provider state to Credential pool key mix-up when multiple custom providers share the same base_url #19083, where cached provider/base-url identity can lead to using the wrong credential path.
• Also relevant to endpoint-selection/fallback patterns discussed around fix(doctor): retry DashScope China endpoint on 401 (salvage #19191) #21248.

Test plan

pytest tests/agent/test_credential_pool.py -q

Result: 46 passed.

[DanielLSM]

Closing in favor of consolidated PR #22827.