Record struct packing even when falling back to auto layout

[jkoritzinsky]

Fixes #116475

[Copilot]

Pull Request Overview

This pull request fixes the record struct packing for cases that fall back to auto layout. Key changes include:

• Adding new tests in StructPacking.cs (TestAction and an overload for Test) to verify expected native sizes.
• Setting the packing size explicitly when using Auto layout in methodtablebuilder.cpp.
• Consolidating the SetPackingSize implementation in class.h by removing a duplicate definition.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/tests/Interop/StructPacking/StructPacking.cs	Adds tests for struct packing validation using different layouts.
src/coreclr/vm/methodtablebuilder.cpp	Explicitly sets the packing size for auto layout cases.
src/coreclr/vm/class.h	Removes the duplicate SetPackingSize to avoid redundancy.

Comments suppressed due to low confidence (2)

src/tests/Interop/StructPacking/StructPacking.cs:139

• The new TestAction method adds tests to validate struct packing behavior when falling back to auto layout. Verify that these tests comprehensively cover the scenarios for both 64-bit and 32-bit processes.

// Test data types that invalidate managed sequential layout

src/coreclr/vm/class.h:463

• The duplicate implementation of SetPackingSize has been removed. Ensure that all external references now use the remaining version without any side effects.

void SetPackingSize(BYTE cbPackingSize)

[jkoritzinsky]

/backport to release/10.0-preview6

[github-actions]

Started backporting to release/10.0-preview6: https://github.com/dotnet/runtime/actions/runs/15720419045

[jkoritzinsky]

/ba-g failures unrelated