fix(input): carry key and preedit text as bytes like Zig

Zig's KeyEvent.utf8 and the embedded API's key/preedit text are byte
slices passed through to key encoding and the pty; the port converted
them lossily to UTF-8.
- KeyEvent.utf8 is &[u8] end to end (ghostty-input encoder, core,
  ghostty-lib C API, libghostty-vt key events, GTK callers). Where Zig
  validates with Utf8View (alternates, modify-other-keys, ctrl CSI-u,
  associated text, binding unicode lookup) invalid text takes the same
  "no UTF-8" fallback.
- Invalid preedit UTF-8 behaves like Zig's `try Utf8View.init`: the
  prior preedit is cleared and marked dirty, the callback returns an
  error (logged by the C API) and no preedit is set.
- The inspector key widget shows invalid key text lossily, display only.

Test Plan:
- cargo test --workspace --no-fail-fast (3407 passed)
- cargo test -p ghostty-lib --features test-platform (C-API key text
  0xff 0xfe reaches the pty unchanged)
- just check-macos (both), just check-windows, just check-wasm

crates/ghostty-apprt-gtk/src/class/surface.rs

@@ -1306,7 +1306,7 @@ impl Surface {
             mods,
             consumed_mods,
             composing: imp.im_composing.get(),
-            utf8: &utf8,
+            utf8: utf8.as_bytes(),
             unshifted_codepoint: keyval_unicode_unshifted,
         }) {
             Ok(e) => e,
@@ -3673,7 +3673,7 @@ mod imp {
             let (text, _, _) = ctx.preedit_string();
 
             // Update our preedit state in Ghostty core
-            if let Err(err) = surface.preedit_callback(Some(&text)) {
+            if let Err(err) = surface.preedit_callback(Some(text.as_bytes())) {
                 log::warn!("error in preedit callback err={err}");
             }
         }
@@ -3755,7 +3755,7 @@ mod imp {
                     mods: Mods::default(),
                     consumed_mods: Mods::default(),
                     composing: false,
-                    utf8: str,
+                    utf8: str.as_bytes(),
                     ..KeyEvent::default()
                 }) {
                     log::warn!("error in key callback err={err}");