feat: make it possible to toggle mouse mode in the Rust TUI (#971)

I did a bit of research to understand why I could not use my mouse to
drag to select text to copy to the clipboard in iTerm.

Apparently https://github.com/openai/codex/pull/641 to enable mousewheel
scrolling broke this functionality. It seems that, unless we put in a
bit of effort, we can have drag-to-select or scrolling, but not both.
Though if you know the trick to hold down `Option` will dragging with
the mouse in iTerm, you can probably get by with this. (I did not know
about this option prior to researching this issue.)

Nevertheless, users may still prefer to disable mouse capture
altogether, so this PR introduces:

* the ability to set `tui.disable_mouse_capture = true` in `config.toml`
to disable mouse capture
* a new command, `/toggle-mouse-mode` to toggle mouse capture

codex-rs/tui/src/slash_command.rs

@@ -13,6 +13,7 @@ use strum_macros::IntoStaticStr;
 #[strum(serialize_all = "kebab-case")]
 pub enum SlashCommand {
     Clear,
+    ToggleMouseMode,
     Quit,
 }
 
@@ -21,6 +22,9 @@ impl SlashCommand {
     pub fn description(self) -> &'static str {
         match self {
             SlashCommand::Clear => "Clear the chat history.",
+            SlashCommand::ToggleMouseMode => {
+                "Toggle mouse mode (enable for scrolling, disable for text selection)"
+            }
             SlashCommand::Quit => "Exit the application.",
         }
     }