[Efficiency Improver] perf: eliminate state-machine allocations in PropertyBag.Single(T)() and SingleOrDefault(T)()

[Evangelink]

Replace triple-LINQ enumeration in Single() and yield-iterator enumeration in SingleOrDefault() with direct linked-list walks.

Single() previously called Any(), Skip(1).Any(), and First() on the same IEnumerable returned by Property.OfType() — allocating up to four separate yield-state-machine objects per call.

SingleOrDefault() previously called Property.OfType() and then GetEnumerator() on the result — allocating one state-machine object per call.

Both methods now walk the Property linked list directly (the same pattern already used by Property.Any() and Property.Contains()), allocating zero heap objects in the common case.

Fixes #7854

[Copilot]

Pull request overview

This PR optimizes PropertyBag.Single<TProperty>() and PropertyBag.SingleOrDefault<TProperty>() in Microsoft.Testing.Platform by removing LINQ/yield-based enumeration paths that were creating iterator state-machine allocations, replacing them with direct linked-list traversal over the internal Property nodes.

Changes:

• Replaced SingleOrDefault<TProperty>()’s OfType<TProperty>() + enumerator usage with a direct linked-list walk that tracks a single match and throws on duplicates.
• Replaced Single<TProperty>()’s multi-enumeration LINQ pattern (Any(), Skip(1).Any(), First()) with a single-pass linked-list walk.
• Added explanatory comments describing the allocation avoidance rationale.

Show a summary per file

File	Description
src/Platform/Microsoft.Testing.Platform/Messages/PropertyBag.cs	Removes iterator/LINQ-based enumeration from Single* APIs and replaces it with allocation-free linked-list traversal.

Copilot's findings

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

[JanKrivanek]

Is this on a hot path? In what scenarios and how often, with how big dataset is this being called?

We have a tradeoff of code brevity + expressiveness Versus perf - let's make sure we prefer perf if it really has impact

[JanKrivanek]

Approving - but please validate this is a change on hot path, with real perf impact