test: replace FluentAssertions with NUnit assertions

[rsubrama83]

Summary

Closes #15. Removes FluentAssertions and uses NUnit's Assert.That throughout, matching the rest of the test suite.

FluentAssertions was only used in 6 places, so this is a small change:

Location	Before	After
FontStyleSetTests	.Should().BeTrue() / BeFalse()	Is.True / Is.False
DynamicImageTests	BeEquivalentTo(new Size(...))	Is.EqualTo (Size is a readonly struct)
TestPlan measure	BeEquivalentTo(expectedMeasure)	Is.EqualTo (SpacePlan is a readonly struct)
TestPlan draw ops	BeEquivalentTo(..., PreferringRuntimeMemberTypes)	Is.EqualTo on the operation list
TextStyleTests	BeEquivalentTo(..., IncludingNestedObjects, ...)	Is.EqualTo (TextStyle is a record, Fallback compared recursively)
GridTests	unused using	removed

OperationBase and its 9 subclasses (test engine only) are now records, which gives them value equality. Record equality also checks the runtime type, so it's slightly stricter than BeEquivalentTo.

Failure messages

• Operations: the record ToString() prints every property, for example
  Expected: <CanvasTranslateOperation { Position = (Left: 1,000, Top: 0,000) }> But was: <... (Left: 0,000 ...)>
• TextStyle: its members are internal, so the record ToString() prints TextStyle { }. The test now passes a message that lists all members of both styles.

Verification

• All tests pass: 150/150 unit tests, 130/130 example tests (forced rebuild)
• Mutation checks, both caught by the new assertions:
  • shifting every expected CanvasTranslateOperation by 1px fails the alignment tests
  • changing the expected Fallback.HasUnderline fails ApplyInheritedAndGlobalStyle

test: commit, so merging does not trigger a release.

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