fix: raise ValidationError for out-of-range int→float in convert - #1129

Summary

msgspec.convert(big_int, float) for integers outside the finite double range leaked a SystemError (<built-in function convert> returned a result with an exception set) instead of raising msgspec.ValidationError.

Root cause: convert_int called PyLong_AsDouble(obj) and passed the result into ms_decode_float without checking PyErr_Occurred(). When the C conversion overflows, Python sets OverflowError but the unchecked path still returns a value, and the interpreter surfaces that as SystemError.

json.decode already reports the same case as ValidationError: Number out of range.

Fix

In convert_int's MS_TYPE_FLOAT branch:

1. Capture PyLong_AsDouble(obj)
2. If x == -1.0 && PyErr_Occurred(), clear the exception and return ms_error_with_path("Number out of range%U", path)
3. Otherwise decode the float as before

Test plan

• pytest tests/unit/test_convert.py::TestFloat -q → 8 passed
• Manual: convert(10**400, float) and nested dict[str, float] raise ValidationError: Number out of range
• Manual: in-range convert(5, float) == 5.0 still works
• Matches json.decode(str(10**400).encode(), type=float) error class/message

Fixes #1122