tui: add feature-gated /plan slash command to switch to Plan mode (#10103)

## Summary
Adds a simple `/plan` slash command in the TUI that switches the active
collaboration mode to Plan mode. The command is only available when the
`collaboration_modes` feature is enabled.

## Changes
- Add `plan_mask` helper in `codex-rs/tui/src/collaboration_modes.rs`
- Add `SlashCommand::Plan` metadata in
`codex-rs/tui/src/slash_command.rs`
- Implement and hard-gate `/plan` dispatch in
`codex-rs/tui/src/chatwidget.rs`
- Hide `/plan` when collaboration modes are disabled in
`codex-rs/tui/src/bottom_pane/slash_commands.rs`
- Update command popup tests in
`codex-rs/tui/src/bottom_pane/command_popup.rs`
- Add a focused unit test for `/plan` in
`codex-rs/tui/src/chatwidget/tests.rs`

## Behavior notes
- `/plan` is now a no-op if `Feature::CollaborationModes` is disabled.
- When enabled, `/plan` switches directly to Plan mode without opening
the picker.

## Codex author
`codex resume 019c05da-d7c3-7322-ae2c-3ca38d0ef702`

codex-rs/tui/src/collaboration_modes.rs

@@ -59,3 +59,7 @@ pub(crate) fn next_mask(
 pub(crate) fn code_mask(models_manager: &ModelsManager) -> Option<CollaborationModeMask> {
     mask_for_kind(models_manager, ModeKind::Code)
 }
+
+pub(crate) fn plan_mask(models_manager: &ModelsManager) -> Option<CollaborationModeMask> {
+    mask_for_kind(models_manager, ModeKind::Plan)
+}