Use issuer URL in device auth prompt link

[abrar71]

What

Update the device-code login flow to build the browser URL and redirect URI from the configured OAuth issuer instead of hard-coding https://auth.openai.com.

Why

When using a custom or experimental issuer (for example via --experimental_issuer), the CLI still shows the default auth.openai.com device URL, which is confusing and can send user to the wrong login page.

How

• Thread issuer_base_url through run_device_code_login into print_device_code_prompt.
• Replace the hard-coded device auth URL with {issuer_base_url}/codex/device in the prompt.
• Use issuer_base_url consistently for api_base_url and the redirect URI ({issuer_base_url}/deviceauth/callback).

Tests

• just fmt
• cargo test -p codex-login

[abrar71]

I have read the CLA Document and I hereby sign the CLA

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[etraut-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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