composebox_typeahead: Fix checking whether messages can be sent.

Before calling `on_enter_send`, we checked if the
`compose-send-button` is disabled, but never actually
set it to disabled, rather we updated its container
(.message-send-controls) with that state.

This commit fixes that by correctly checking the button's
container for the class .disabled-message-send-controls
which is set to indicate that the user cannot send messages
at the moment.

Fixes #30620

web/src/composebox_typeahead.ts

@@ -360,7 +360,7 @@ function handle_keydown(
                     e.preventDefault();
                     if (
                         compose_validate.validate_message_length() &&
-                        !$("#compose-send-button").prop("disabled")
+                        !$(".message-send-controls").hasClass("disabled-message-send-controls")
                     ) {
                         on_enter_send();
                     }