Add tests for nullable numeric types CanBeNumeric property

[konard]

Summary

• Added comprehensive test cases for nullable numeric types (int?, double?, bool?) to verify they are correctly marked as CanBeNumeric
• Verified that the existing implementation already handles nullable numeric types correctly
• The NumericType<T> class properly extracts the underlying type for nullable types and checks if that type can be numeric

Test plan

• Added unit tests for nullable numeric types in NumericTypeTests.cs
• Verified all existing tests still pass (12 total tests)
• Created experimental code to demonstrate the functionality works as expected
• Confirmed nullable numeric types (int?, double?, etc.) return CanBeNumeric: True
• Confirmed non-numeric nullable types (string?) return CanBeNumeric: False

Analysis

The issue was already resolved in the current implementation. The NumericType<T> constructor correctly:

1. Detects if a type is nullable using type.IsNullable()
2. Extracts the underlying type using Nullable.GetUnderlyingType(type)
3. Checks if the underlying type can be numeric using underlyingType.CanBeNumeric()

This means int? becomes int, double? becomes double, and so on, and since these underlying types are numeric, the nullable versions are correctly marked as CanBeNumeric.

Fixes #22

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