Show action required in terminal title (#18372)

Implements #18162

This updates the TUI terminal title to show an explicit action-required
state when Codex is blocked on user approval or input. The terminal
title now uses the activity title item to cover both active work and
blocked-on-user states, while still accepting the legacy spinner config
value.

Changes
- Rename the terminal title item from `spinner` to `activity` while
preserving legacy config compatibility
- Show `[ ! ] Action Required `while approval or input overlays are
active, with a blinking `[ . ]` alternate state
- Suppress the normal working spinner while Codex is blocked on user
action
- Add targeted coverage for action-required title behavior and legacy
title-item parsing

Testing
- Trigger an approval or input modal and confirm the tab title
alternates between `[ ! ] Action Required` and `[ . ] Action Required`
- Disable the activity title item and confirm the action-required title
does not appear
- Resolve the prompt and confirm the title returns to the normal
spinning/idel state


https://github.com/user-attachments/assets/e9ecc530-a6be-4fd7-b9a6-d550a790eb2c

codex-rs/core/src/config/mod.rs

@@ -455,7 +455,9 @@ pub struct Config {
 
     /// Ordered list of terminal title item identifiers for the TUI.
     ///
-    /// When unset, the TUI defaults to: `project` and `spinner`.
+    /// When unset, the TUI defaults to: `activity` and `project`.
+    /// The `activity` item spins while working and shows an action-required
+    /// message when blocked on the user.
     pub tui_terminal_title: Option<Vec<String>>,
 
     /// Syntax highlighting theme override (kebab-case name).