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/mcp-server/tests/common/mcp_process.rs

@@ -18,6 +18,7 @@ use codex_protocol::mcp_protocol::CancelLoginChatGptParams;
 use codex_protocol::mcp_protocol::GetAuthStatusParams;
 use codex_protocol::mcp_protocol::InterruptConversationParams;
 use codex_protocol::mcp_protocol::ListConversationsParams;
+use codex_protocol::mcp_protocol::LoginApiKeyParams;
 use codex_protocol::mcp_protocol::NewConversationParams;
 use codex_protocol::mcp_protocol::RemoveConversationListenerParams;
 use codex_protocol::mcp_protocol::ResumeConversationParams;
@@ -318,6 +319,15 @@ impl McpProcess {
         self.send_request("resumeConversation", params).await
     }
 
+    /// Send a `loginApiKey` JSON-RPC request.
+    pub async fn send_login_api_key_request(
+        &mut self,
+        params: LoginApiKeyParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("loginApiKey", params).await
+    }
+
     /// Send a `loginChatGpt` JSON-RPC request.
     pub async fn send_login_chat_gpt_request(&mut self) -> anyhow::Result<i64> {
         self.send_request("loginChatGpt", None).await