Fix toasts on Windows under WSL 2 (#7137)

Before this: no notifications or toasts when using Codex CLI in WSL 2.

After this: I get toasts from Codex

codex-rs/tui/src/notifications/osc9.rs

@@ -0,0 +1,37 @@
+use std::fmt;
+use std::io;
+use std::io::stdout;
+
+use crossterm::Command;
+use ratatui::crossterm::execute;
+
+#[derive(Debug, Default)]
+pub struct Osc9Backend;
+
+impl Osc9Backend {
+    pub fn notify(&mut self, message: &str) -> io::Result<()> {
+        execute!(stdout(), PostNotification(message.to_string()))
+    }
+}
+
+/// Command that emits an OSC 9 desktop notification with a message.
+#[derive(Debug, Clone)]
+pub struct PostNotification(pub String);
+
+impl Command for PostNotification {
+    fn write_ansi(&self, f: &mut impl fmt::Write) -> fmt::Result {
+        write!(f, "\x1b]9;{}\x07", self.0)
+    }
+
+    #[cfg(windows)]
+    fn execute_winapi(&self) -> io::Result<()> {
+        Err(std::io::Error::other(
+            "tried to execute PostNotification using WinAPI; use ANSI instead",
+        ))
+    }
+
+    #[cfg(windows)]
+    fn is_ansi_code_supported(&self) -> bool {
+        true
+    }
+}