Fix regression breaking self-insert of kitty shifted codepoint

Commit 50a6e486a5 (Allow explicit shift modifier for non-ASCII
letters, fix capslock behavior, 2025-03-30) delayed handling of kitty
keyboard protocol's shifted codepoints.  It does handle shifted
codepoints when matching keys to mappings; but it fails to handle
them in the self-insert code paths where we want to insert the text
represented by CharEvent::Key.
Fix it by resolving the shifted key.

Fixes #11813

(cherry picked from commit bb916f8d73)

CHANGELOG.rst

@@ -1,3 +1,10 @@
+fish 4.0.9 (released ???)
+=========================
+
+This release fixes a regression in 4.0.6 that caused shifted keys to not be inserted on some terminals.
+
+--------------
+
 fish 4.0.8 (released September 18, 2025)
 ========================================
 

src/input_common.rs

@@ -171,10 +171,15 @@ pub fn from_single_byte(c: u8) -> Self {
     }
 
     pub(crate) fn codepoint_text(&self) -> Option<char> {
-        if self.modifiers.is_some() {
+        let mut modifiers = self.modifiers;
+        let mut c = self.codepoint;
+        if self.shifted_codepoint != '\0' && modifiers.shift {
+            modifiers.shift = false;
+            c = self.shifted_codepoint;
+        }
+        if modifiers.is_some() {
             return None;
         }
-        let c = self.codepoint;
         if c == key::Space {
             return Some(' ');
         }