Summary

• add KIMI_CN_API_KEY to doctor's provider env hints
• make the Kimi / Moonshot (China) doctor probe null-safe when base_env is not set
• add regression coverage for both behaviors

Why

hermes doctor appears to have two doctor-only issues for the kimi-coding-cn provider:

1. KIMI_CN_API_KEY is not included in doctor's provider env hints, so China-only Kimi setups can be misclassified as missing provider credentials
2. the Kimi China probe entry uses base_env=None, but the later probe loop still calls os.getenv(_base_env, "") unconditionally, which can raise TypeError: str expected, not NoneType

This PR keeps the fix intentionally narrow and limits it to the diagnostic layer in hermes_cli/doctor.py.

Fixes #9716

Tests

I added regression coverage in tests/hermes_cli/test_doctor.py for:

• recognizing KIMI_CN_API_KEY as a configured provider env
• ensuring the Kimi China doctor probe no longer crashes when base_env is None

Local verification:

• python -m pytest -o addopts='' tests/hermes_cli/test_doctor.py -q
• result: 19 passed

The repository's default pytest configuration includes -n, which is not available in this local environment, so I ran the same test file with addopts disabled.