fix Rngs.__getattr__ returning default stream for dunder names (#5375)

[mohsinm-dev]

Fixes #5375

Rngs.__getattr__ falls back to returning the default RngStream for any attribute name not explicitly
defined, including dunder names like __pydantic_serializer__. This breaks any library that probes for
protocol support via hasattr/getattr on dunder names.

Added an early guard in __getattr__ to raise AttributeError for dunder names so only user-chosen
stream names trigger the fallback.

if name.startswith('__') and name.endswith('__'):
    raise AttributeError(name)