[profile-switcher][rust] -- [1/2] Add app-server account session protocol (#25469)

## Summary

Adds the app-server v2 `accountSession/*` protocol used by the Desktop
profile switcher and the backend account metadata client needed to
populate workspace choices.

This is the protocol layer only. The app-server lifecycle and
consolidated saved-session storage are split into a follow-up PR.

## Rust Stack

1. This PR
2. [openai/codex#25383](https://github.com/openai/codex/pull/25383) adds
app-server session lifecycle behavior and consolidated saved-session
storage.

## Validation

- Generated app-server schema fixtures are included from the existing
generation flow in the lifecycle PR where the routes are registered.
- Did not run tests per requested scope.

codex-rs/backend-client/src/lib.rs

@@ -4,6 +4,8 @@ pub(crate) mod types;
 pub use client::AddCreditsNudgeCreditType;
 pub use client::Client;
 pub use client::RequestError;
+pub use types::AccountEntry;
+pub use types::AccountsCheckResponse;
 pub use types::CodeTaskDetailsResponse;
 pub use types::CodeTaskDetailsResponseExt;
 pub use types::ConfigBundleResponse;