Add subscript setter support with handler and call count

[farkasseb]

Summary

• Generate functional subscript setters with SetCallCount and SetHandler instead of the previous empty set { } stub
• Distinguish { get } and { get set } subscripts correctly; get-only subscripts now omit the setter entirely
• Support both regular and concurrency-safe mocks (Sendable and actor mocks) using the same MockoloMutex / MockoloHandlerState pattern as getter handling

Design decisions

No built-in value store for setters

The generated setter handler receives all subscript parameters plus newValue, but it does not persist values automatically.

A generic built-in store is hard to make sound:

• not all subscript key or value types are Hashable
• generic subscript signatures can be complex
• Any-erased storage would lose type safety

Instead, storage can be added explicitly in tests when needed:

var storage: [Int: String] = [:]
mock.subscriptSetHandler = { key, newValue in storage[key] = newValue }
mock.subscriptHandler = { key in storage[key] ?? "" }

This keeps the generated code minimal and leaves storage semantics under the test author's control.

Current limitation

Protocol refinement is not reconciled across inheritance. If a parent protocol declares a { get set } subscript and a child narrows it to { get }, the generated mock follows the accessor block on the protocol being mocked rather than merging inherited accessor requirements.

Behavioral changes

Previously, all subscripts, including { get }-only subscripts, were generated with an empty set { } block. That made get-only mocks writable and diverged from the protocol contract.

Now:

• { get } subscripts generate only a getter
• { get set } subscripts generate a functional setter with SetCallCount and SetHandler

The main compatibility impact is that code which wrote to a get-only mock subscript will now fail to compile, which is the correct behavior.

[sidepelican]

Nice catch. Thank you!