The _diagnose_connectivity() function previously only probed google.com
to determine whether the user has internet access. Since google.com is
blocked by China's Great Firewall, Chinese users were always misdiagnosed
as having no internet, causing misleading LocalNetworkError messages.
Now checks the Comfy API health endpoint first (the most relevant
signal), then falls back to multiple probe URLs (google.com, baidu.com,
captive.apple.com) to support users in regions where specific sites are
blocked.
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