fix(credential_pool,commands): recover Z.AI exhausted entries and register /costs command

[DanielLSM]

Summary

This PR fixes two independent but related issues:

1. Z.AI credential pool exhaustion recovery: When a Z.AI pool entry is marked exhausted after a 429, the fallback path uses the provider registry's default base_url (the regular endpoint). For accounts with quota only on the coding-plan endpoint, this causes permanent failure even though the coding endpoint still works.

2. Missing /costs command registration: The usage command was registered but not costs or credits, causing Telegram/gateway to not recognize /costs as a slash command.

Changes

agent/credential_pool.py

• Add _sync_zai_entry_from_auth_store(): when a Z.AI entry is exhausted, check provider_state.zai.detected_endpoint in auth.json. If runtime resolution detected a different working endpoint, adopt it and clear exhaustion immediately.
• Re-probe on cooldown expiry: when clear_expired resets an exhausted Z.AI entry after its cooldown, call detect_zai_endpoint() to verify the cached base_url is still valid. If the endpoint changed, update it. If no endpoint works, keep the entry exhausted (preserving the original error context).
• Mirrors the existing sync patterns for anthropic, nous, and openai-codex.

hermes_cli/commands.py

• Register costs as the canonical command with usage and credits as aliases.
• Update should_bypass_active_session comment to reference costs.

cli.py, gateway/run.py

• Update dispatch handlers to check canonical == "costs" instead of "usage".

Tests

• tests/agent/test_credential_pool.py: add 3 regression tests for Z.AI endpoint recovery, re-probe on expiry, and failed re-probe keeping entry unavailable.
• tests/hermes_cli/test_commands.py: add regression tests for costs canonical resolution, gateway known commands, and Telegram menu exposure.

Test Plan

python -m pytest tests/agent/test_credential_pool.py -q
python -m pytest tests/hermes_cli/test_commands.py -q

All tests pass locally.

Related

• Fixes credential pool exhaustion causing wrong Z.AI endpoint fallback.
• Fixes /costs slash command not being recognized by Telegram/gateway.

[DanielLSM]

Closing in favor of #21413 which generalizes this fix and references related issues more comprehensively.