Simplify auth flow and reconcile differences between ChatGPT and API Key auth (#3189)

This PR does the following:
* Adds the ability to paste or type an API key.
* Removes the `preferred_auth_method` config option. The last login
method is always persisted in auth.json, so this isn't needed.
* If OPENAI_API_KEY env variable is defined, the value is used to
prepopulate the new UI. The env variable is otherwise ignored by the
CLI.
* Adds a new MCP server entry point "login_api_key" so we can implement
this same API key behavior for the VS Code extension.
<img width="473" height="140" alt="Screenshot 2025-09-04 at 3 51 04 PM"
src="https://github.com/user-attachments/assets/c11bbd5b-8a4d-4d71-90fd-34130460f9d9"
/>
<img width="726" height="254" alt="Screenshot 2025-09-04 at 3 51 32 PM"
src="https://github.com/user-attachments/assets/6cc76b34-309a-4387-acbc-15ee5c756db9"
/>

codex-rs/core/src/model_provider_info.rs

@@ -80,7 +80,10 @@ pub struct ModelProviderInfo {
     /// the connection as lost.
     pub stream_idle_timeout_ms: Option<u64>,
 
-    /// Whether this provider requires some form of standard authentication (API key, ChatGPT token).
+    /// Does this provider require an OpenAI API Key or ChatGPT login token? If true,
+    /// user is presented with login screen on first run, and login preference and token/key
+    /// are stored in auth.json. If false (which is the default), login screen is skipped,
+    /// and API key (if needed) comes from the "env_key" environment variable.
     #[serde(default)]
     pub requires_openai_auth: bool,
 }