Refactor Assert.That into focused partial-class files

[Copilot]

Assert.That.cs had grown into a 1.5K-line mixed-responsibility file, making the assertion pipeline hard to navigate. This change keeps API/behavior intact and splits internal logic into focused partial-class files under Assertions/.

• Public API surface kept in Assert.That.cs

  • Retained namespace/class declaration, constants, failure sentinel, and the public Assert.That(...) entry point.
  • Removed internal helper implementations from this file so it is now API-focused.

• Expression evaluation extracted

  • Added Assert.That.ExpressionEvaluator.cs.
  • Moved recursive expression-walk/evaluation methods (including short-circuit, coalesce, and conditional handling).

• Expression replacement extracted

  • Added Assert.That.ExpressionReplacer.cs.
  • Moved sub-expression replacement helpers used to simplify expression rendering.

• Failure-detail extraction extracted

  • Added Assert.That.DetailExtractor.cs.
  • Moved variable/member/method/index detail collection and sentinel translation helpers.

• Expression formatting extracted

  • Added Assert.That.ExpressionFormatter.cs.
  • Moved expression-text cleanup and value-formatting helpers, including compiler-generated wrapper cleanup utilities.
  • IsFuncOrActionType and CompilerGeneratedDisplayClassRegex remain private helpers.

// Assert.That.cs (entry point only)
public static partial class AssertExtensions
{
    public static void That(Expression<Func<bool>> assertion, string? message = null) { ... }
}

// Assert.That.ExpressionEvaluator.cs
public static partial class AssertExtensions
{
    private static object? EvaluateExpression(Expression expression, Dictionary<Expression, object?> cache) { ... }
}

[Copilot]

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