Reduce enumerator boxing in ProjectElementContainer

[Erarndt]

Fixes #

Context

Changes Made

Testing

Notes

[Copilot]

Pull Request Overview

This pull request aims to reduce boxing in the enumerators used within ProjectElementContainer by returning a custom struct Enumerator instead of the interface type.

• Returns a custom Enumerator type for GetEnumerator() in ProjectElementContainer
• Adjusts explicit interface implementations to work with the new Enumerator type

Comments suppressed due to low confidence (2)

src/Build/Construction/ProjectElementContainer.cs:861

• Ensure that the custom Enumerator struct correctly implements IEnumerator to maintain compatibility with consumers of the IEnumerable interface.

public Enumerator GetEnumerator() => new Enumerator(_initial, _forwards);

src/Build/Construction/ProjectElementContainer.cs:953

• Verify that returning the custom Enumerator directly meets all expected IEnumerator behaviors when used in explicit interface implementations.

public readonly Enumerator GetEnumerator() => _enumerator;