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-core/src/surface/keyboard.rs

@@ -92,7 +92,7 @@ pub struct OwnedKeyEvent {
     pub mods: Mods,
     pub consumed_mods: Mods,
     pub composing: bool,
-    pub utf8: String,
+    pub utf8: Vec<u8>,
     pub unshifted_codepoint: u32,
 }
 
@@ -112,7 +112,7 @@ impl OwnedKeyEvent {
             mods: e.mods,
             consumed_mods: e.consumed_mods,
             composing: e.composing,
-            utf8: e.utf8.to_owned(),
+            utf8: e.utf8.to_vec(),
             unshifted_codepoint: e.unshifted_codepoint,
         }
     }
@@ -1067,7 +1067,7 @@ mod tests {
                 ctrl: true,
                 ..Mods::default()
             },
-            utf8: "a",
+            utf8: "a".as_bytes(),
             unshifted_codepoint: 'a' as u32,
             ..KeyEvent::default()
         };