feat: Add spy! macro and #[autospy] attribute

[exoego]

Closes #567

Adds

• spy! procedural macro (analogous to mock!)
• #[autospy] attribute macro (analogous to #[automock]) supports both traits and struct impl blocks
• real.into_spy() helper which is a shorthand for SpyFoo::new(real)

Design decisions:

1. Spy reuses the existing mock pipeline. Instead of a separate code path, spy generation goes through the same MockItemStruct / MockFunction / MockTrait with a GenerateMode flag. This keeps code duplication low but ties spy behavior to mock internals.
2. Trait spies are generic: SpyFoo<T: MyTrait> accepts any type that implements the trait. Struct spies use a concrete type directly because inherent methods have no trait to use as a bound.
3. impl Trait return-types (including plain async fn) are not supported. Because Mockall converts these to Pin<Box<dyn Trait>>, which causes a type mismatch when calling the real implementation.
   • #[async_trait] works because it does this conversion before mockall processes the trait. This is documented as a known limitation.
4. calling_real() is needed for verified delegation. Unlike Java (Mockito)/Ruby(RSpec) spy frameworks that record all calls and let you check them after the fact, Mockall requires expectations to be set up before the call. So, users must explicitly use calling_real() when they want to both delegate and verify.