docs: add troubleshooting for ChatGPT token exchange 403 unsupported region errors

[gaoguobin]

Motivation

• Users reported a ChatGPT login failure showing Token exchange failed ... 403 Forbidden: Country, region, or territory not supported even though the proxy reported provider auth split and needs_restart=false.
• The intent is to document that OAuth/token-exchange failures can be caused by system/WebView routing (VPN/TUN or route switchers) rather than the proxy/provider auth alone.

Description

• Added a troubleshooting block to README.md that documents the Token exchange failed ... 403 Forbidden: Country, region, or territory not supported error.
• Instructed users to verify proxy readiness including needs_restart=false and that provider auth preparation/split completed.
• Advised users to ensure their system proxy/VPN (including TUN mode) captures Codex App/WebView login traffic and to temporarily disable route/proxy switchers such as ccswitch.
• Clarified that this is an OpenAI OAuth region/routing rejection separate from third-party provider auth failures.

Testing

• No automated unit tests were run because this is a documentation-only change and does not affect runtime code or behavior.

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Codex Task