Consider Replacing Moq with NSubstitute

[rvanmaanen]

Context

All test projects currently use Moq for mocking. TechHubApiClient is intentionally non-sealed to support Moq's proxy generation. Moq had the SponsorLink controversy and requires non-sealed classes.

NSubstitute has a cleaner API and works directly with interfaces (no need for non-sealed classes).

Target State

Replace Moq with NSubstitute across all test projects.

Implementation

1. Replace Moq package with NSubstitute in Directory.Packages.props
2. Rewrite mock setups — API comparison:

Moq	NSubstitute
var mock = new Mock<IFoo>();	var sub = Substitute.For<IFoo>();
mock.Setup(x => x.Method()).Returns(val)	sub.Method().Returns(val)
mock.Object	sub (used directly)
mock.Verify(x => x.Method(), Times.Once)	sub.Received(1).Method()

3. TechHubApiClient can be sealed once Moq is removed (tests mock the interface, not the class)

Notes

• Large refactor — consider doing incrementally: new tests use NSubstitute, migrate old tests over time
• Low urgency; only do when there's a natural opportunity

Priority

Low (non-urgent)