feat: migrate template test project from xUnit to TUnit

[Copilot]

Migrates the Coalesce Vue template's test project from xUnit to TUnit, a modern testing framework with source-generated test discovery and native .NET 10 support.

Changes

• Package references: Replace xunit, xunit.runner.visualstudio, Microsoft.NET.Test.Sdk with TUnit (v1.13.56)
• Test syntax: Convert [Fact] → [Test], synchronous methods → async Task
• Assertions: Migrate to TUnit's fluent async assertions (Assert.That(x).IsEqualTo(y))
• AssertionExtensions: Refactor helper methods to use TUnit's fluent API with conditional .Because() clauses
• global.json: Add test.runner: "Microsoft.Testing.Platform" for .NET 10 compatibility

Example

Before:

[Fact]
public void Test1()
{
    Assert.Equal(expected, actual);
}

After:

[Test]
public async Task Test1()
{
    await Assert.That(actual).IsEqualTo(expected);
}

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.