Add theme-aware semantic colors to extension UI trees (#60019)

## Why

Extension UI trees previously accepted only named terminal colors and literal RGB values. Semantic colors let extension content follow the active syntax theme and terminal color capabilities.

## What changed

- Accept `primary`, `surface`, `accent`, and `status-*` color roles for text foregrounds and row backgrounds.
- Resolve semantic colors against the active theme when rendering extension footer content. Prefer the `codex.accent` scope for `accent`, falling back to the theme's accent setting.
- Reuse status-line scope mappings and fallback colors, with terminal-default fallbacks for unavailable primary, surface, and accent colors.

## Testing

Add color serialization coverage and rendering snapshots for dark, light, and empty themes in truecolor and ANSI 16-color terminals, including nested foreground and background styles.

GitOrigin-RevId: 156f137859d74178f9ae6aa46a02eecb815e9802

codex-rs/tui/src/render/highlight.rs

@@ -290,11 +290,16 @@ pub(crate) fn syntax_theme_revision() -> u64 {
 
 /// Clone the current syntax theme (e.g. to save for cancel-restore).
 pub(crate) fn current_syntax_theme() -> Theme {
+    with_syntax_theme(Theme::clone)
+}
+
+/// Borrow the active theme for one synchronous render operation.
+pub(crate) fn with_syntax_theme<T>(f: impl FnOnce(&Theme) -> T) -> T {
     let active_theme = theme_lock();
-    match active_theme.read() {
-        Ok(theme) => theme.clone(),
-        Err(poisoned) => poisoned.into_inner().clone(),
-    }
+    let theme = active_theme
+        .read()
+        .unwrap_or_else(std::sync::PoisonError::into_inner);
+    f(&theme)
 }
 
 /// An explicit diff background, including the ANSI terminal-default marker.
@@ -366,11 +371,18 @@ pub(crate) fn foreground_style_for_scopes_with_theme(
     theme: &Theme,
     scope_names: &[&str],
 ) -> Option<Style> {
-    let highlighter = Highlighter::new(theme);
+    foreground_color_for_scopes(&Highlighter::new(theme), scope_names)
+        .map(|fg| Style::default().fg(fg))
+}
+
+pub(crate) fn foreground_color_for_scopes(
+    highlighter: &Highlighter<'_>,
+    scope_names: &[&str],
+) -> Option<RtColor> {
     scope_names.iter().find_map(|scope_name| {
         let scope = Scope::new(scope_name).ok()?;
         let fg = highlighter.style_mod_for_stack(&[scope]).foreground?;
-        convert_syntect_color(fg).map(|fg| Style::default().fg(fg))
+        convert_syntect_color(fg)
     })
 }