[Maintenance] Add actor and expansion error tests for @_MockedProperty macro

[graycampbell]

What area needs maintenance?

@_MockedProperty macro test coverage in Tests/MockingMacrosTests/Macros/MockedPropertyMacro/.

Why is this improvement needed?

When subscript support was added in PR #148, @_MockedSubscript gained actor expansion tests (MockedSubscript_ActorTests.swift) verifying nonisolated modifiers are generated correctly when isMockAnActor: true, and expansion error tests (MockedSubscriptMacro_ExpansionErrorTests.swift) verifying the macro emits a diagnostic when applied to a non-subscript declaration. @_MockedProperty has neither. The existing assertMockedProperty helper also hard-codes isMockAnActor: false, so actor scenarios cannot be tested at all.

Proposed solution

1. Add an isMockAnActor: Bool = false parameter to assertMockedProperty in Tests/MockingMacrosTests/Macros/MockedPropertyMacro/TestHelpers/AssertMockedProperty.swift
2. Add Tests/MockingMacrosTests/Macros/MockedPropertyMacro/MockedProperty_ActorTests.swift testing read-only and read-write property expansion in actor mocks
3. Add Tests/MockingMacrosTests/Macros/MockedPropertyMacro/MockedPropertyMacro_ExpansionErrorTests.swift testing the error when applied to a non-property declaration