fuzzing: don't return 1 from bb_fuzzer on unexpected exceptions (fixes #1224)

libFuzzer only accepts 0 and -1 from LLVMFuzzerTestOneInput; any other
non-zero value is treated as -1 and the input is dropped from the corpus.
nlohmann parse/type errors are not in ExceptionFilter, so most inputs took
this path: on the seed corpus only 1 of 5 files was kept (cov 650); with
the fall-through to `return 0` all 5 are kept (cov 1607). The message is
still printed. The other two fuzzers already return 0 on every path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: facontidavide

fuzzing/bb_fuzzer.cpp

@@ -263,7 +263,6 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     if(!ExceptionFilter::isExpectedException(e))
     {
       std::cerr << "Unexpected top-level exception: " << e.what() << std::endl;
-      return 1;
     }
   }