fix: preserve Wei and Gwei types in arithmetic

[samsamtrum]

What was wrong?

Wei and Gwei were NewType aliases over int. Static type checkers treat arithmetic on a NewType as returning the base int, so common accumulation patterns lose their unit type and augmented assignment is rejected.

For example:

value = Wei(5)
value += Wei(1)  # mypy sees the expression as int

How was it fixed?

This changes Wei and Gwei to lightweight int subclasses backed by a shared internal _IntegerType helper. The helper preserves the concrete subclass for common integer arithmetic operations such as addition, subtraction, multiplication, floor division, modulo, divmod, and unary operations.

Because the classes still subclass int, existing runtime behavior remains compatible with code that expects integers.

Verification

• python3 -m py_compile web3/types.py tests/core/types/test_currency_types.py
• mypy web3/types.py tests/core/types/test_currency_types.py
• pytest tests/core/types/test_currency_types.py -q

Closes #3833

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve non-integer arithmetic results or raise TypeError

The overridden arithmetic methods coerce every result back through self.__class__(...), which silently truncates non-integer numeric results instead of preserving Python’s normal semantics. For example, Wei(1) + 0.9 now becomes Wei(1) (via int(1.9)), and Wei(1) + Decimal('1.9') similarly drops the fractional part. This is a runtime correctness regression introduced by the new _IntegerType implementation and can corrupt values whenever callers mix Wei/Gwei with non-int numeric types.

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