Move TUI app tests to modules they cover (#18799)

## Summary

The TUI app refactor in #18753 moved the old `app.rs` tests into a
single `app/tests.rs` file. That kept the split mechanically simple, but
it left several focused unit tests far from the modules they exercise.

This PR is a follow-up that moves tests next to the code they cover.

It also adds `tui/src/app/test_support.rs` for shared fixture
construction.

This is just a mechanical refactoring (no functional changes) and does
not affect any production code.

codex-rs/tui/src/app.rs

@@ -200,8 +200,6 @@ mod thread_session_state;
 use self::agent_navigation::AgentNavigationDirection;
 use self::agent_navigation::AgentNavigationState;
 use self::app_server_requests::PendingAppServerRequests;
-#[cfg(test)]
-use self::background_requests::*;
 use self::loaded_threads::find_loaded_subagent_threads_for_primary;
 use self::pending_interactive_replay::PendingInteractiveReplayState;
 use self::platform_actions::*;
@@ -1152,5 +1150,7 @@ impl Drop for App {
     }
 }
 
+#[cfg(test)]
+pub(super) mod test_support;
 #[cfg(test)]
 mod tests;