[Feature] Support @Mocked on protocols with subscripts

[gage-halverson-fetch]

Use Case

I'm trying to create mocks for protocols that use subscripts for data access patterns (like caches, stores, dictionaries). Currently, @Mocked doesn't support protocols with subscript requirements.

Feature Proposal

I would like to see @Mocked support protocols with subscripts. For example:

@Mocked
protocol DataStore {
    subscript(key: String) -> String? { get set }
}

Then the interface could look something like this

// In your test
let mock = DataStoreMock()

// Setting up the getter to return a value
mock._subscriptGet.implementation = .returns("cached value")

// Or with logic
mock._subscriptGet.implementation = .invokes { key in
    if key == "user_id" {
        return "12345"
    }
    return nil
}

// Setting up the setter
mock._subscriptSet.implementation = .invokes { key, value in
    print("Set \(key) to \(value)")
}

// Using it
let value = mock["user_id"]  // Returns "12345"
mock["name"] = "John"        // Invokes the setter

Added complexity with multiple subscripts with different symbols.

Alternatives Considered

Use OSAllocatedUnfairLock inside Swift 6 packages.

Additional Context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[graycampbell]

Since subscript is similar to a property (just without the let or var keyword and with the addition of the key), the interface for this would look similar to the interface for a property:

mock._subscript.getter.implementation = .invokes { key in "value" }
mock._subscript.setter.implementation = .invokes { key, value in }