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[stdlib] Enforce non-NULL PythonObjects to prevent segfaults

This implements safer error handling for Python-Mojo bindings to avoid
segfaults from NULL pointer propagation.

Changes:
- Added PythonException type for explicit error representation
- Changed PyFunctionRaising to return Variant[PythonException, PythonObject]
- Added NULL kwargs canonicalization (NULL -> empty dict)
- Added NULL check in PythonObject.__del__ to prevent segfaults
- Added comprehensive tests for exception handling and NULL safety

Fixes modular#5414
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Signed-off-by: Dalton Alexandre <166029845+dl-alexandre@users.noreply.github.com>