Fix mode 2026 feature detection response

The response to the 2026 mode query was missing a `?` character.

The response should be of the format `CSI ? 2026 ; N $ y` where N can
be any value in the range 0-4 inclusive.

References:

https://gist.github.com/christianparpart/d8a62cc1ab659194337d73e399004036
https://vt100.net/docs/vt510-rm/DECRPM.html

zellij-server/src/panes/grid.rs

@@ -2745,7 +2745,7 @@ impl Perform for Grid {
                 for param in params_iter.map(|param| param[0]) {
                     match param {
                         2026 => {
-                            let response = "\u{1b}[2026;2$y";
+                            let response = "\u{1b}[?2026;2$y";
                             self.pending_messages_to_pty
                                 .push(response.as_bytes().to_vec());
                         },