Add surrogate pair exception to json superset fuzzer

Surrogate pairs are not supported by RCL on purpose, so when that can be
parsed by Serde but is rejected by RCL, we shouldn't fail the fuzzer on
it.

fuzz/src/uber.rs

@@ -243,12 +243,18 @@ fn fuzz_eval_json_superset(loader: &mut Loader, input: &str) {
         Ok(..) => {
             // Works as intended.
         }
-        Err(err) if format!("{err:?}").contains("Overflow") => {
-            // In cases of a large integer or large exponent, Serde parses it
-            // into a different data type or into a float that goes Inf, but RCL
-            // cannot represent those, so the incompatibility here is
-            // intentional.
+        Err(err) => {
+            let msg = format!("{err:?}");
+            if msg.contains("Overflow") {
+                // In cases of a large integer or large exponent, Serde parses
+                // it into a different data type or into a float that goes Inf,
+                // but RCL cannot represent those, so the incompatibility here
+                // is intentional.
+            } else if msg.contains("not a Unicode scalar value") {
+                // RCL does not support surrogate pairs on purpose.
+            } else {
+                panic!("If Serde can parse it, RCL should be able to, but got: {msg}");
+            }
         }
-        Err(err) => panic!("If Serde can parse it, RCL should be able to, but got: {err:?}"),
     }
 }