Fix: Incorrect casting when assigning Enum to custom type

- Modified TypeConversionAnalyzer.cs to use EnumCastThenConversion for all non-enum types, ensuring that the necessary cast through the underlying type is generated (e.g. `(MyType)(int)MyEnum.Value1`).
- Added unit test Issue1211_EnumToCustomTypeImplicitConversionAsync to ensure regressions don't occur.

Co-authored-by: GrahamTheCoder <2490482+GrahamTheCoder@users.noreply.github.com>

Author: google-labs-jules[bot]