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-lib/src/embedded.rs

@@ -216,7 +216,7 @@ struct OwnedKey {
     mods: Mods,
     consumed_mods: Mods,
     composing: bool,
-    text: String,
+    text: Vec<u8>,
     unshifted_codepoint: u32,
 }
 
@@ -234,7 +234,7 @@ impl OwnedKey {
         };
         // SAFETY: per the function contract.
         let text = unsafe { cstr(e.text) }
-            .map(|c| c.to_string_lossy().into_owned())
+            .map(|c| c.to_bytes().to_vec())
             .unwrap_or_default();
 
         // We want to get the physical unmapped key to process keybinds.
@@ -1636,7 +1636,7 @@ pub unsafe extern "C" fn ghostty_surface_preedit(
     } else {
         // SAFETY: `text` points to `len` readable bytes.
         let bytes = unsafe { std::slice::from_raw_parts(text.cast::<u8>(), len) };
-        Some(String::from_utf8_lossy(bytes).into_owned())
+        Some(bytes.to_vec())
     };
     if let Err(err) = core.preedit_callback(preedit.as_deref()) {
         log::error!("error in preedit callback err={err}");