Refactor BoolExtensions into dedicated BooleanAssertions type

[Copilot]

Promotes boolean assertions from extension methods on ObjectAssertions<bool> to a dedicated BooleanAssertions class, matching the IntegerAssertions<T> pattern.

Changes

• BooleanAssertions — New sealed class inheriting ObjectAssertions<bool> with BeTrue(), NotBeTrue(), BeFalse(), NotBeFalse() returning BooleanAssertionsChain
• BooleanAssertionsChain — New readonly struct with And and Value properties for fluent chaining
• ShouldExtensions — Added Should(this bool) overload with [OverloadResolutionPriority(1)] returning BooleanAssertions
• BooleanExtensions — Stripped to only nullable bool? overloads (still extension methods on ObjectAssertions<bool?>)
• Tests — Split into BooleanAssertionsTests (non-nullable) and BooleanExtensionsTests (nullable); added chaining and inherited method tests
• Docs — Updated tree, class docs, and method docs to reflect new structure

Before/After

// Before: extension method returns ObjectAssertionsChain<bool>
true.Should().BeTrue().And.Equal(true);

// After: instance method returns BooleanAssertionsChain, .And gives BooleanAssertions
true.Should().BeTrue().And.NotEqual(false);
//   ^^^^^^^^           ^^^
//   BooleanAssertions  BooleanAssertions (inherits ObjectAssertions<bool>)

Also fixed a pre-existing bug in NotBeTrue's error message — it previously said "was false" when the value was actually true.

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