Refactor Dumper.dump for safety and cleaner output

[insign]

Refactored Dumper.dump in lib/src/dumper.dart to use a try-finally block for managing the _visited set. This ensures that objects are always removed from the set, even if an exception occurs during processing, preventing potential issues with recursion detection.

Also improved the fallback output for objects that do not have a toJson method. Instead of wrapping them in a redundant { ... } block with the class name, the dumper now directly outputs the result of toString(), resulting in cleaner and more readable debug output.

Updated test/var_dump_test.dart to reflect the improved output format for non-JSON objects.

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PR created automatically by Jules for task 17756405774714712785 started by @insign

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