Move test assertions to AwesomeAssertions

[Copilot]

Replaces TUnit's built-in Assert.That() assertions with AwesomeAssertions for better test reporting.

Changes

• tests/Directory.Build.props — Added AwesomeAssertions 9.4.* package reference
• All test files — Migrated assertions from TUnit's await Assert.That(x).IsEqualTo(y) to AwesomeAssertions' fluent .Should().Be(y) style

Before

await Assert.That(result.IsValid).IsFalse();
await Assert.That(result.Errors[0].Code).IsEqualTo(ValidationErrorCode.InvalidXml);
await Assert.That(result.Errors[0].Message).Contains("Invalid XML");

After

result.IsValid.Should().BeFalse();
result.Errors[0].Code.Should().Be(ValidationErrorCode.InvalidXml);
result.Errors[0].Message.Should().Contain("Invalid XML");

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