Handle missing plan info for ChatGPT accounts (#13072)

Addresses https://github.com/openai/codex/issues/13007 and
https://github.com/openai/codex/issues/12170

There are situations where the ChatGPT auth backend might return a JWT
that contains no plan information. Most code paths already handle this
case well, but the internal implementation of the "account/read" app
server call was failing in this case (returning an error rather than
properly returning None for the plan).

This resulted in a situation where users needed to log in every time the
extension or app started even if they successfully logged in the last
time.

Summary
- allow ChatGPT-authenticated accounts to fall back to
`AccountPlanType::Unknown` when the token omits the plan claim
- add regression coverage in `app-server/tests/suite/v2/account.rs` to
confirm `account/read` returns `plan_type: Unknown` when the claim is
absent
- ensure the Rust auth helpers and fixtures treat missing plan claims as
Optional and default to `Unknown`

codex-rs/app-server/tests/suite/v2/account.rs

@@ -1227,3 +1227,45 @@ async fn get_account_with_chatgpt() -> Result<()> {
     assert_eq!(received, expected);
     Ok(())
 }
+
+#[tokio::test]
+async fn get_account_with_chatgpt_missing_plan_claim_returns_unknown() -> Result<()> {
+    let codex_home = TempDir::new()?;
+    create_config_toml(
+        codex_home.path(),
+        CreateConfigTomlParams {
+            requires_openai_auth: Some(true),
+            ..Default::default()
+        },
+    )?;
+    write_chatgpt_auth(
+        codex_home.path(),
+        ChatGptAuthFixture::new("access-chatgpt").email("user@example.com"),
+        AuthCredentialsStoreMode::File,
+    )?;
+
+    let mut mcp = McpProcess::new_with_env(codex_home.path(), &[("OPENAI_API_KEY", None)]).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let params = GetAccountParams {
+        refresh_token: false,
+    };
+    let request_id = mcp.send_get_account_request(params).await?;
+
+    let resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(request_id)),
+    )
+    .await??;
+    let received: GetAccountResponse = to_response(resp)?;
+
+    let expected = GetAccountResponse {
+        account: Some(Account::Chatgpt {
+            email: "user@example.com".to_string(),
+            plan_type: AccountPlanType::Unknown,
+        }),
+        requires_openai_auth: true,
+    };
+    assert_eq!(received, expected);
+    Ok(())
+}