.LastItem().Satisfies() loses specialised assertion source for item

[thomhurst]

Mirror of #5706 for the LastItem path. Surfaced in code review of PR #5764.

Problem

ListLastItemSource<TList, TItem>.Satisfies(...) and ReadOnlyListLastItemSource<TList, TItem>.Satisfies(...) still pass IAssertionSource<TItem> (a generic fallback) into the lambda. When TItem is a string, collection, dictionary, or set, specialised assertion methods are unreachable — same shape as #5706.

Repro

IList<List<int>> nestedLists = new List<List<int>> { new() { 1, 2, 3 } };

// Fails: item is IAssertionSource<List<int>>, no .Count() in scope
await Assert.That(nestedLists).LastItem().Satisfies(item => item.Count().IsEqualTo(3));

// IList<string>
IList<string> strings = new List<string> { "alpha" };
await Assert.That(strings).LastItem().Satisfies(s => s.Contains("foo"));

Affected signatures

• TUnit.Assertions/Conditions/ListAssertions.cs:250 — ListLastItemSource<TList, TItem>.Satisfies(Func<IAssertionSource<TItem>, Assertion<TItem>?>, ...)
• TUnit.Assertions/Conditions/ReadOnlyListAssertions.cs (mirror) — ReadOnlyListLastItemSource<TList, TItem>.Satisfies(...)

Proposed fix

Apply the same IAssertionSourceFor<TItem, TSelf> pattern PR #5764 introduced for ItemAt:

1. Add a generic Satisfies<TSource>(Func<TSource, IAssertion?>, ...) instance method on ListLastItemSource and ReadOnlyListLastItemSource, constrained where TSource : IAssertionSourceFor<TItem, TSource>. Wrap in #if !NETSTANDARD2_0.
2. Add an internal CreateSatisfiesAssertion(Func<TItem, IAssertion?>, string?) helper alongside, mirroring ListItemAtSource.CreateSatisfiesAssertion.
3. Add a parallel ListLastItemSatisfiesExtensions (or extend the existing item-at extensions file) with the same per-shape thin delegators — interface-shaped overloads delegate to Satisfies<TSource>, concrete List<T>/Dictionary<K,V> overloads work via the same dual IAssertionSourceFor impls already on ListAssertion and MutableDictionaryAssertion.
4. Mirror the Issue5706Tests set against LastItem() to keep coverage parity.

Reference

PR #5764 — same pattern applied to ItemAt. Files to model after:

• Generic dispatch + internal helper: TUnit.Assertions/Conditions/ListAssertions.cs (Satisfies on ListItemAtSource)
• Per-shape delegators: TUnit.Assertions/Extensions/ListItemAtSatisfiesExtensions.cs
• Tests: TUnit.Assertions.Tests/Bugs/Issue5706Tests.cs

Acceptance

• Specialised assertions reachable inside .LastItem().Satisfies(...) lambda for string/collection/dict/set items.
• Same on the ReadOnlyListLastItemSource path.
• Tests cover each specialised element shape (interface and concrete).

[thomhurst]

Will be addressed in #5764 directly rather than a separate PR.