Fix traceback, syntax errors, and exception handling by youknowone · Pull Request #7015 · RustPython/RustPython

- Improve unclosed bracket detection with "'(' was never closed" message
- Fix IndentationError location to point to end of line
- Implement frame.clear() with proper checks for executing/suspended frames
- Fix exception context chaining for propagated exceptions
- Add traceback.__dir__() and prevent tb_next deletion
- Fix subscript operation source range restoration in compiler
- Change "duplicate parameter" to "duplicate argument" error message
- Refactor duplicate code in asyncgenerator.rs and frame.rs

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