Add safety hint for parse_marshal_inplace

Signed-off-by: Dude so hot <djohn@fbi.gov>

app/src/main/rust/src/lib.rs

@@ -110,6 +110,7 @@ where
     jni_throwing(env, |env| {
         let buffer = deref_mut_direct_bytebuffer(env, str)?;
         let value = {
+            // SAFETY: ktor client ensure html content is valid utf-8.
             let html = unsafe { from_utf8_unchecked(&buffer[..limit as usize]) };
             let dom = tl::parse(html, ParserOptions::default()).map_err(|e| anyhow!(e))?;
             ensure!(dom.version().is_some(), "{html}");