fix(agent): clarify auxiliary OAuth provider credential errors

[rasitakyol]

Summary

• Add credential-type-aware errors when explicit auxiliary provider client creation returns no client.
• Mention Vertex ADC/OAuth/service-account setup and google-auth instead of suggesting a fake VERTEX_API_KEY.
• Preserve existing API-key provider guidance for API-key-backed providers.

Fixes #56810

Tests

• scripts/run_tests.sh tests/agent/test_auxiliary_client.py -k 'test_vertex_unavailable_mentions_adc_oauth_not_fake_api_key' -v --tb=short (RED before fix: failed on VERTEX_API_KEY)
• scripts/run_tests.sh tests/agent/test_auxiliary_client.py -k 'TestExplicitProviderUnavailableMessages' -v --tb=short (2 passed)
• scripts/run_tests.sh tests/agent/test_auxiliary_client.py -v --tb=short (283 passed)
• git diff --check

[alt-glitch]

This was generated by AI during triage.

Duplicate of #56842 (earliest of the three competing fixes for #56810). This PR and #56842 use the same mechanism at the same two raise sites: a helper that keys off the provider's auth_type in PROVIDER_REGISTRY and replaces the hardcoded <X>_API_KEY RuntimeError with an auth-mechanism-aware message. #56843 is the other duplicate of #56842. Maintainer picks one; #56842 is canonical as the earliest.

[rasitakyol]

Thanks for the triage — closing this in favor of #56842.