Move TUI on top of app server (parallel code) (#14717)

This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.

Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.

codex-rs/tui_app_server/src/collaboration_modes.rs

@@ -0,0 +1,62 @@
+use codex_protocol::config_types::CollaborationModeMask;
+use codex_protocol::config_types::ModeKind;
+
+use crate::model_catalog::ModelCatalog;
+
+fn filtered_presets(model_catalog: &ModelCatalog) -> Vec<CollaborationModeMask> {
+    model_catalog
+        .list_collaboration_modes()
+        .into_iter()
+        .filter(|mask| mask.mode.is_some_and(ModeKind::is_tui_visible))
+        .collect()
+}
+
+pub(crate) fn presets_for_tui(model_catalog: &ModelCatalog) -> Vec<CollaborationModeMask> {
+    filtered_presets(model_catalog)
+}
+
+pub(crate) fn default_mask(model_catalog: &ModelCatalog) -> Option<CollaborationModeMask> {
+    let presets = filtered_presets(model_catalog);
+    presets
+        .iter()
+        .find(|mask| mask.mode == Some(ModeKind::Default))
+        .cloned()
+        .or_else(|| presets.into_iter().next())
+}
+
+pub(crate) fn mask_for_kind(
+    model_catalog: &ModelCatalog,
+    kind: ModeKind,
+) -> Option<CollaborationModeMask> {
+    if !kind.is_tui_visible() {
+        return None;
+    }
+    filtered_presets(model_catalog)
+        .into_iter()
+        .find(|mask| mask.mode == Some(kind))
+}
+
+/// Cycle to the next collaboration mode preset in list order.
+pub(crate) fn next_mask(
+    model_catalog: &ModelCatalog,
+    current: Option<&CollaborationModeMask>,
+) -> Option<CollaborationModeMask> {
+    let presets = filtered_presets(model_catalog);
+    if presets.is_empty() {
+        return None;
+    }
+    let current_kind = current.and_then(|mask| mask.mode);
+    let next_index = presets
+        .iter()
+        .position(|mask| mask.mode == current_kind)
+        .map_or(0, |idx| (idx + 1) % presets.len());
+    presets.get(next_index).cloned()
+}
+
+pub(crate) fn default_mode_mask(model_catalog: &ModelCatalog) -> Option<CollaborationModeMask> {
+    mask_for_kind(model_catalog, ModeKind::Default)
+}
+
+pub(crate) fn plan_mask(model_catalog: &ModelCatalog) -> Option<CollaborationModeMask> {
+    mask_for_kind(model_catalog, ModeKind::Plan)
+}