Replace FluentAssertions with Shouldly

[phmatray]

Summary

• Replaced FluentAssertions NuGet package (8.8.0) with Shouldly (4.3.0) in both test projects
• Updated all GlobalUsings.cs files to use Shouldly instead of FluentAssertions
• Converted all assertion patterns across 41 test files (667 insertions, 687 deletions)
• All 455 tests pass after migration (384 unit tests + 71 GitHub tests)

Key Assertion Mappings

FluentAssertions	Shouldly
.Should().Be(x)	.ShouldBe(x)
.Should().BeTrue()	.ShouldBeTrue()
.Should().NotBeNull()	.ShouldNotBeNull()
.Should().Throw<T>().WithMessage(m)	Should.Throw<T>(action); ex.Message.ShouldBe(m)
.Should().ThrowAsync<T>()	Should.ThrowAsync<T>(action)
.Should().ContainEquivalentOf(x)	.ShouldContain(x)
.Should().BeCloseTo(dt, ts)	(DateTime.Now - dt).ShouldBeLessThan(ts)
.As<T>()	.ShouldBeAssignableTo<T>() or cast

Test plan

• All 384 unit tests pass on net10.0
• All 71 GitHub integration tests pass on net10.0
• No remaining FluentAssertions references in any test file
• No remaining .Should() calls in any test file

Fixes #134