Fix bytes to string conversion to explicitly error on invalid code points

Fixes #716

PiperOrigin-RevId: 768102889

Author: l46kok

runtime/src/main/java/dev/cel/runtime/CelStandardFunctions.java

@@ -890,7 +890,18 @@ public enum Conversions implements StandardOverload {
         BYTES_TO_STRING(
             (bindingHelper) ->
                 CelFunctionBinding.from(
-                    "bytes_to_string", ByteString.class, ByteString::toStringUtf8)),
+                    "bytes_to_string",
+                    ByteString.class,
+                    (byteStr) -> {
+                      if (!byteStr.isValidUtf8()) {
+                        throw new CelRuntimeException(
+                            new IllegalArgumentException(
+                                "invalid UTF-8 in bytes, cannot convert to string"),
+                            CelErrorCode.BAD_FORMAT);
+                      }
+
+                      return byteStr.toStringUtf8();
+                    })),
         TIMESTAMP_TO_STRING(
             (bindingHelper) ->
                 CelFunctionBinding.from(

runtime/src/test/resources/stringConversions_error.baseline

@@ -0,0 +1,5 @@
+Source: string(b'\xff')
+=====>
+bindings: {}
+error:    evaluation error at test_location:6: invalid UTF-8 in bytes, cannot convert to string
+error_code:    BAD_FORMAT

testing/src/main/java/dev/cel/testing/BaseInterpreterTest.java

@@ -1679,6 +1679,12 @@ public void stringConversions() {
     runTest();
   }
 
+  @Test
+  public void stringConversions_error() throws Exception {
+    source = "string(b'\\xff')";
+    runTest();
+  }
+
   @Test
   public void bytes() throws Exception {
     source =