fix: inconsistent behavior from legacy and kitty Key encoding (#5728)

this fix addresses issue occurring in nvim/vim where Backspace deletes
previous letter while in preedit state.

related to #1638

> same issue reproducible
> However, that only fixed for legacy encoding.

Unanswered Discussion
https://github.com/ghostty-org/ghostty/discussions/5312<div
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tested on macos sequioa IME '2-set Korean', 'Japanese Romaji'

src/input/KeyEncoder.zig

@@ -101,11 +101,14 @@ fn kitty(
 
         // IME confirmation still sends an enter key so if we have enter
         // and UTF8 text we just send it directly since we assume that is
-        // whats happening.
+        // whats happening. See legacy()'s similar logic for more details
-        if (self.event.key == .enter and
+        // on how to verify this.
-            self.event.utf8.len > 0)
+        if (self.event.utf8.len > 0) {
-        {
+            switch (self.event.key) {
-            return try copyToBuf(buf, self.event.utf8);
+                .enter => return try copyToBuf(buf, self.event.utf8),
+                .backspace => return "",
+                else => {},
+            }
         }
 
         // If we're reporting all then we always send CSI sequences.
@@ -1723,6 +1726,27 @@ test "kitty: keypad number" {
     try testing.expectEqualStrings("[57400;;49u", actual[1..]);
 }
 
+test "kitty: backspace with utf8 (dead key state)" {
+    var buf: [128]u8 = undefined;
+    var enc: KeyEncoder = .{
+        .event = .{
+            .key = .backspace,
+            .utf8 = "A",
+            .unshifted_codepoint = 0x0D,
+        },
+        .kitty_flags = .{
+            .disambiguate = true,
+            .report_events = true,
+            .report_alternates = true,
+            .report_all = true,
+            .report_associated = true,
+        },
+    };
+
+    const actual = try enc.kitty(&buf);
+    try testing.expectEqualStrings("", actual);
+}
+
 test "legacy: backspace with utf8 (dead key state)" {
     var buf: [128]u8 = undefined;
     var enc: KeyEncoder = .{