Add [StackTraceHidden] to assertion API internals

[Evangelink]

Addresses feedback from @Youssef1313 on #8826 (#8826 (comment)):

Why not use StackTraceHidden instead of trying to alter stack trace manually.

This PR adds [StackTraceHidden] to nested helper types inside the assertion API that previously could leak into user-facing failure stack traces:

• NonGenericEqualityComparerAdapter (Assert.AreAllDistinct)
• TypeMismatch (Assert.AreAllOfType)
• EquivalenceComparer partial (Assert.AreEquivalent.Comparer and sibling partials)
• DictionaryView, DictionaryLookup, NonGenericDictionaryView, GenericDictionaryView, GenericDictionaryAccessors (Assert.AreEquivalent.DictionaryView)
• ReferenceObjectComparer, MemberLookup, MemberAccessor (Assert.AreEquivalent.MemberAccessor)
• EquivalenceMismatch (Assert.AreEquivalent.Mismatch)
• ThrowsExceptionState (Assert.ThrowsException)
• ObjectComparer (CollectionAssert.Helpers)

Top-level assertion types (Assert, CollectionAssert, StringAssert, AssertExtensions, AssertScope, etc.) and all InterpolatedStringHandler structs were already annotated, so this PR fills in the remaining nested helpers within src/TestFramework/TestFramework/Assertions/.

Enum-typed helpers (IEquatableOutcome, ThrowsFailureKind) are skipped because [StackTraceHidden] does not target enums.

Notes / out of scope

• MTP / adapter plumbing frames that may also appear in user failure traces are out of scope for this PR.
• Compiler-generated iterator/lambda state-machine frames inside annotated types may still appear in some cases — that is a known runtime limitation of [StackTraceHidden] and not addressed here.

[Copilot]

Pull request overview

This PR extends [StackTraceHidden] coverage within the MSTest assertion implementation (src/TestFramework/TestFramework/Assertions/*) by annotating additional nested/internal helper types that can otherwise appear in user-facing failure stack traces.

Changes:

• Add [StackTraceHidden] to additional nested assertion helper types in Assert.AreEquivalent.* (comparer, mismatch, member accessor, dictionary view).
• Add [StackTraceHidden] to helper types used by Assert.AreAllDistinct, Assert.AreAllOfType, and Assert.ThrowsException.
• Add [StackTraceHidden] to CollectionAssert’s internal ObjectComparer.

Show a summary per file

File	Description
src/TestFramework/TestFramework/Assertions/CollectionAssert.Helpers.cs	Hides ObjectComparer helper frames from stack traces.
src/TestFramework/TestFramework/Assertions/Assert.ThrowsException.cs	Hides ThrowsExceptionState helper frames from stack traces.
src/TestFramework/TestFramework/Assertions/Assert.AreEquivalent.Mismatch.cs	Hides EquivalenceMismatch helper frames from stack traces.
src/TestFramework/TestFramework/Assertions/Assert.AreEquivalent.MemberAccessor.cs	Hides member-access and reference-compare helper frames from stack traces.
src/TestFramework/TestFramework/Assertions/Assert.AreEquivalent.DictionaryView.cs	Hides dictionary abstraction/lookup/accessor helper frames from stack traces.
src/TestFramework/TestFramework/Assertions/Assert.AreEquivalent.Comparer.cs	Hides EquivalenceComparer internal comparer frames from stack traces.
src/TestFramework/TestFramework/Assertions/Assert.AreAllOfType.cs	Hides TypeMismatch helper frames from stack traces.
src/TestFramework/TestFramework/Assertions/Assert.AreAllDistinct.cs	Hides comparer adapter frames from stack traces.

Copilot's findings

• Files reviewed: 8/8 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 8/8 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 8/8 changed files
• Comments generated: 1