Rename tui_app_server to tui (#16104)

This is a follow-up to https://github.com/openai/codex/pull/15922. That
previous PR deleted the old `tui` directory and left the new
`tui_app_server` directory in place. This PR renames `tui_app_server` to
`tui` and fixes up all references.

codex-rs/tui/tests/test_backend.rs

@@ -0,0 +1,4 @@
+#[path = "../src/test_backend.rs"]
+mod inner;
+
+pub use inner::VT100Backend;