fix(auth): treat empty credential pool entries as unauthenticated in /model picker

[rudi193-cmd]

Summary

Fixes #28140.

The /model picker checked whether a provider key existed in credential_pool, not whether it had any entries. An empty list ("minimax-cn": []) left after removing an API key from .env still caused the provider to appear as selectable.

hermes_cli/model_switch.py line 1235 — one-line change:

# before — key presence is enough to pass
if store and hermes_id in store.get("credential_pool", {}):

# after — requires at least one non-empty entry
if store and store.get("credential_pool", {}).get(hermes_id):

Test plan

• Configure a provider via hermes model, then delete its API key from .env
• Confirm ~/.hermes/auth.json still has "provider-id": [] in credential_pool
• Run /model — provider should no longer appear as available

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