#[derive(Decode)] changes to make generics more robust

[orthecreedence]

This allows #[derive(Decode)] to be used in the following cases:

enum MyType<T> {
    // has no T
    Variant1(String),
    // uses T directly
    Variant2(T),
    // uses T indirectly
    Variant3(Vec<T>),
    // indirect T on struct variant
    VariantStruct4 {
	val: Vec<T>,
    }
}

Before, the <T> generic would be applied and expected to exist on ALL enum variants, causing compile errors when deriving.

Tests updated.

I feel it is my duty to disclose I did use AI assistance in this update. Derive macros and the intricacies of the supporting libraries are not my strong suit. I reviewed the result fully to make sure it's not doing anything "weird" but it's possible the implementation is overly verbose or incorrect in some obvious way. Tests are passing, but some additional scrutiny might be a good idea.

[XAMPPRocky]

Thank you for your PR!