test: capture try_read panic on multi-byte input

Author: SAY-5

src/reader.rs

@@ -457,4 +457,19 @@ mod tests {
         assert_eq!(None, r.try_read());
         assert_eq!(None, r.try_read());
     }
+
+    // Multi-byte UTF-8 characters in the buffer cause `drain(..1)` to panic at
+    // the char boundary. Marked `should_panic` to capture the current behavior;
+    // the follow-up fix commit removes the attribute and asserts correct decode.
+    #[test]
+    #[should_panic = "is_char_boundary"]
+    fn test_try_read_multibyte() {
+        let f = io::Cursor::new("\u{c3}");
+        let mut r = NBReader::new(f, Options::default());
+        // pump bytes from the reader thread into the buffer
+        for _ in 0..10 {
+            let _ = r.try_read();
+            thread::sleep(time::Duration::from_millis(5));
+        }
+    }
 }