Use issuer URL in device auth prompt link

[abrar71]

Summary

When using device-code login with a custom issuer (--experimental_issuer), Codex correctly uses that issuer for the auth flow — but the terminal prompt still told users to open the default OpenAI device URL (https://auth.openai.com/codex/device). That’s confusing and can send users to the wrong domain (especially for enterprise/staging issuers). This PR updates the prompt (and related URLs) to consistently use the configured issuer. 🎯

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🔧 What changed

• 🔗 Device auth prompt link now uses the configured issuer (instead of a hard-coded OpenAI URL)
• 🧭 Redirect callback URL is derived from the same issuer for consistency
• 🧼 Minor cleanup: normalize the issuer base URL once and reuse it (avoids formatting quirks like trailing /)

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🧪 Repro + Before/After

▶️ Command

codex login --device-auth --experimental_issuer https://auth.example.com

❌ Before (wrong link shown)

1. Open this link in your browser and sign in to your account
   https://auth.openai.com/codex/device

✅ After (correct link shown)

1. Open this link in your browser and sign in to your account
   https://auth.example.com/codex/device

Full example output (same as before, but with the correct URL):

Welcome to Codex [v0.72.0]
OpenAI's command-line coding agent

Follow these steps to sign in with ChatGPT using device code authorization:

1. Open this link in your browser and sign in to your account
   https://auth.example.com/codex/device

2. Enter this one-time code (expires in 15 minutes)
   BUT6-0M8K4

Device codes are a common phishing target. Never share this code.

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✅ Test plan

• 🟦 codex login --device-auth (default issuer): output remains unchanged

• 🟩 codex login --device-auth --experimental_issuer https://auth.example.com:

  • prompt link points to the issuer ✅
  • callback URL is derived from the same issuer ✅
  • no double slashes / mismatched domains ✅

[abrar71]

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

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[etraut-openai]

@codex review

[chatgpt-codex-connector]

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

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[abrar71]

@etraut-openai Happy New Year, Could you please review and merge this small change. Thank you!

[etraut-openai]

@abrar71, Happy New Year to you too. Yes, this is in the queue. The developer who is most familiar with this code was busy with other higher-priority work prior to the holiday break. We'll get to it soon.

[mzeng-openai]

LGTM! This is very helpful, please make sure tests pass before you merge.