fix(auth): set active_provider after hermes auth add google-gemini-cli

[AhmetArif0]

Problem

hermes auth add google-gemini-cli called pool.add_entry() but never wrote
to providers["google-gemini-cli"] or set active_provider in auth.json.

_model_section_has_credentials() checks get_active_provider() first. With
active_provider unset and no api_key_env_vars configured for an
oauth_external provider, the setup wizard reported "No inference provider
configured" even after a successful OAuth login.

Root cause

Same pattern as openai-codex (fixed in #37517) and xai-oauth (fixed in
#37576): pool.add_entry() only writes the credential-pool entry; it doesn't
touch providers[provider] or active_provider in auth.json.

Runtime credential resolution for google-gemini-cli reads tokens directly from
the Google OAuth credential file via agent.google_oauth.get_valid_access_token().
So the agent itself worked fine — only the setup-wizard detection was broken.

Fix

Add _save_google_gemini_cli_tokens() in hermes_cli/auth.py that writes a
minimal provider-state singleton and calls _save_provider_state(), which sets
active_provider. Call it in auth_commands before pool.add_entry().

The pool.add_entry() call is kept so hermes auth list continues to show the
entry. Runtime resolution continues to use agent.google_oauth directly; the
auth.json singleton is only used for provider detection.

Test plan

• New test test_auth_add_google_gemini_cli_sets_active_provider passes
• All 47 test_auth_commands tests pass (46 upstream + 1 new)