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-vt-capi/src/key_event.rs

@@ -27,8 +27,8 @@ pub struct KeyEventWrapper {
 }
 
 impl KeyEventWrapper {
-    /// Borrow as the Rust [`KeyEvent`]. Zig's `utf8` is a byte slice, ours is
-    /// `&str`: invalid UTF-8 is truncated to its longest valid prefix.
+    /// Borrow as the Rust [`KeyEvent`]. `utf8` is a byte slice (as in Zig) and is
+    /// passed through unchanged, even if it is not valid UTF-8.
     ///
     /// # Safety
     /// The caller-provided UTF-8 bytes must still be alive.
@@ -39,17 +39,13 @@ impl KeyEventWrapper {
             // SAFETY: per the contract of `ghostty_key_event_set_utf8`.
             unsafe { std::slice::from_raw_parts(self.utf8, self.utf8_len) }
         };
-        let text = match std::str::from_utf8(bytes) {
-            Ok(s) => s,
-            Err(e) => std::str::from_utf8(&bytes[..e.valid_up_to()]).unwrap_or(""),
-        };
         KeyEvent {
             action: self.action,
             key: self.key,
             mods: self.mods,
             consumed_mods: self.consumed_mods,
             composing: self.composing,
-            utf8: text,
+            utf8: bytes,
             unshifted_codepoint: self.unshifted_codepoint,
         }
     }

crates/ghostty-vt-capi/tests/wasm_examples.rs

@@ -282,7 +282,7 @@ fn wasm_key_encode_example() {
             action,
             key,
             mods,
-            utf8,
+            utf8: utf8.as_bytes(),
             unshifted_codepoint: unshifted as u32,
             ..KeyEvent::default()
         };