Clarify device-code phishing warning (#31648)

## Why

The existing device-code warning does not help users distinguish a login
they initiated from a phishing attempt. The warning should tell users to
stop when the code came from a website or another person.

## What changed

- Updated the warning in the direct CLI and TUI device-code login flows
with actionable guidance.
- Added focused coverage for the styled direct CLI prompt.

codex-rs/login/src/device_code_auth_tests.rs

@@ -0,0 +1,10 @@
+use super::*;
+
+#[test]
+fn device_code_prompt_renders_phishing_warning() {
+    let prompt = device_code_prompt("https://example.com/device", "ABCD-EFGH");
+
+    assert!(prompt.contains(
+        "\x1b[90mContinue only if you started this login in Codex. If a website or another person gave you this code, cancel.\x1b[0m"
+    ));
+}