test(tui2): re-enable ANSI for VT100 tests (#8423)

Codex Unified Exec injects NO_COLOR=1 (and TERM=dumb) into shell tool
commands to keep output stable. Crossterm respects NO_COLOR and
suppresses ANSI escapes, which breaks our VT100-backed tests that assert
on parsed ANSI color output (they see vt100::Color::Default everywhere).

Force ANSI color output back on in the VT100 test backend by overriding
crossterm's memoized NO_COLOR setting in VT100Backend::new. This keeps
Unified Exec behavior unchanged while making the VT100 tests meaningful
and deterministic under Codex.

> [!WARNING]  
> it's possible that this might be a race condition problem for this and
need to be solved a different way. Feel free to revert if it causes the
opposite problem for other tests that assume NOCOLOR is set. If it does
then we need to probably add some extra AGENTS.md lines for how to run
tests when using unified exec.

(this same change was made in tui, so it's probably safe).

codex-rs/tui2/src/test_backend.rs

@@ -25,6 +25,7 @@ pub struct VT100Backend {
 impl VT100Backend {
     /// Creates a new `TestBackend` with the specified width and height.
     pub fn new(width: u16, height: u16) -> Self {
+        crossterm::style::Colored::set_ansi_color_disabled(false);
         Self {
             crossterm_backend: CrosstermBackend::new(vt100::Parser::new(height, width, 0)),
         }