feat(mocks): replace MockSetupBuilder + MockVerifyBuilder with unifie…

…d MockMembersBuilder

The new MockMembersBuilder generates a single _MockMembers.g.cs file per
mocked type, replacing the separate _MockSetup.g.cs and _MockVerify.g.cs
files. Key changes:

- Extension methods target Mock<T> directly instead of IMockSetup<T>
- No more holder classes (_MockSetup, _MockVerify) — engine accessed via mock.Engine
- Typed wrappers are now sealed classes (not structs) with lazy setup registration
- Each wrapper implements ICallVerification for unified setup + verify
- Non-qualifying methods return MockMethodCall<T> or VoidMockMethodCall base types
- Member name conflict detection appends '_' for Mock<T> built-in names
- Property extensions use PropertyMockCall<T> targeting Mock<T>