Use issuer URL in device auth prompt link (#7858)

## 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. 🎯

---

## 🔧 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 `/`)

---

## 🧪 Repro + Before/After

### ▶️ Command

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

### ❌ Before (wrong link shown)

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

### ✅ After (correct link shown)

```text
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):

```text
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.
```

---

## ✅ 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 ✅

Co-authored-by: Eric Traut <etraut@openai.com>

codex-rs/login/src/device_code_auth.rs

@@ -137,26 +137,27 @@ async fn poll_for_token(
     }
 }
 
-fn print_device_code_prompt(code: &str) {
+fn print_device_code_prompt(code: &str, issuer_base_url: &str) {
     println!(
         "\nWelcome to Codex [v{ANSI_GRAY}{version}{ANSI_RESET}]\n{ANSI_GRAY}OpenAI's command-line coding agent{ANSI_RESET}\n\
 \nFollow these steps to sign in with ChatGPT using device code authorization:\n\
-\n1. Open this link in your browser and sign in to your account\n   {ANSI_BLUE}https://auth.openai.com/codex/device{ANSI_RESET}\n\
+\n1. Open this link in your browser and sign in to your account\n   {ANSI_BLUE}{issuer_base_url}/codex/device{ANSI_RESET}\n\
 \n2. Enter this one-time code {ANSI_GRAY}(expires in 15 minutes){ANSI_RESET}\n   {ANSI_BLUE}{code}{ANSI_RESET}\n\
 \n{ANSI_GRAY}Device codes are a common phishing target. Never share this code.{ANSI_RESET}\n",
         version = env!("CARGO_PKG_VERSION"),
-        code = code
+        code = code,
+        issuer_base_url = issuer_base_url
     );
 }
 
 /// Full device code login flow.
 pub async fn run_device_code_login(opts: ServerOptions) -> std::io::Result<()> {
     let client = reqwest::Client::new();
-    let base_url = opts.issuer.trim_end_matches('/');
+    let issuer_base_url = opts.issuer.trim_end_matches('/');
-    let api_base_url = format!("{}/api/accounts", opts.issuer.trim_end_matches('/'));
+    let api_base_url = format!("{issuer_base_url}/api/accounts");
     let uc = request_user_code(&client, &api_base_url, &opts.client_id).await?;
 
-    print_device_code_prompt(&uc.user_code);
+    print_device_code_prompt(&uc.user_code, issuer_base_url);
 
     let code_resp = poll_for_token(
         &client,
@@ -171,10 +172,10 @@ pub async fn run_device_code_login(opts: ServerOptions) -> std::io::Result<()> {
         code_verifier: code_resp.code_verifier,
         code_challenge: code_resp.code_challenge,
     };
-    let redirect_uri = format!("{base_url}/deviceauth/callback");
+    let redirect_uri = format!("{issuer_base_url}/deviceauth/callback");
 
     let tokens = crate::server::exchange_code_for_tokens(
-        base_url,
+        issuer_base_url,
         &opts.client_id,
         &redirect_uri,
         &pkce,