Fix size calculation for derived explicit layout types with no fields

[jkoritzinsky]

Fixes #118196

[Copilot]

Pull Request Overview

This PR fixes a bug in size calculation for derived explicit layout types that have no fields of their own. The issue was that when a class with explicit layout derives from another explicit layout class but doesn't define any fields, the size calculation was incorrectly starting from 0 instead of the parent class size.

Key changes:

• Initialize size calculation with parent class size instead of 0 in classlayoutinfo.cpp
• Add comprehensive test cases covering explicit layout inheritance scenarios

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/vm/classlayoutinfo.cpp	Fix size calculation initialization to use parent size instead of 0
src/tests/JIT/Methodical/structs/ExplicitLayout.cs	Add test classes and methods to verify explicit layout inheritance behavior