Add tests for assembly resolution downgrading via extension mechanisms

[Copilot]

This PR adds comprehensive tests to validate that assembly resolution extension mechanisms can successfully downgrade assembly version requests, while normal runtime resolution cannot.

cc @dotnet/appmodel @AaronRobinsonMSFT

What was missing

The .NET runtime supports assembly version downgrading (resolving a request for A, Version=3.0 with A, Version=1.0) but only through extension mechanisms:

• AppDomain.AssemblyResolve event handler
• AssemblyLoadContext.Resolving event handler
• Custom AssemblyLoadContext.Load override

However, there were no tests specifically validating this downgrading behavior.

What this PR adds

New test assemblies:

• System.Runtime.Loader.Test.AssemblyVersion1 - produces assembly with version 1.0.0

New test class AssemblyResolutionDowngradeTest with 4 test methods:

1. AppDomainAssemblyResolve_CanDowngradeVersion - Validates that AppDomain.AssemblyResolve can resolve a version 3.0.0 request with version 1.0.0 assembly
2. AssemblyLoadContextResolving_CanDowngradeVersion - Validates that AssemblyLoadContext.Resolving event can perform the same downgrade
3. CustomAssemblyLoadContextLoad_CanDowngradeVersion - Validates that custom AssemblyLoadContext.Load override can perform downgrades
4. NormalResolution_CannotDowngradeVersion - Validates baseline behavior that normal runtime resolution cannot downgrade versions

Test approach

Each test:

• Requests assembly System.Runtime.Loader.Test.VersionDowngrade, Version=3.0.0.0
• Extension mechanism returns the version 1.0.0 assembly instead
• Verifies the runtime accepts the downgrade and returns the 1.0.0 assembly
• Validates the returned assembly works correctly (can invoke methods, correct version reported)

The tests use RemoteExecutor to isolate assembly loading behavior and prevent interference between test runs.

Fixes #84039.

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[Copilot]

Pull Request Overview

This PR adds comprehensive tests to validate that assembly resolution extension mechanisms can successfully downgrade assembly version requests from higher to lower versions, while normal runtime resolution cannot perform such downgrades.

Key changes:

• Creates test infrastructure with two test assemblies having the same name but different versions (1.0.0 and 3.0.0)
• Adds test methods validating downgrade behavior across three extension mechanisms: AppDomain.AssemblyResolve, AssemblyLoadContext.Resolving, and custom AssemblyLoadContext.Load override
• Includes platform-specific testing for normal resolution behavior differences between CoreCLR and Mono

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
System.Runtime.Loader.Tests.csproj	Adds the new test class and references to the version test assemblies as embedded resources
AssemblyResolutionDowngradeTest.cs	Main test class with four test methods validating downgrade scenarios and helper classes
System.Runtime.Loader.Test.AssemblyVersion1.csproj	Test assembly project configured for version 1.0.0 with shared assembly name
VersionTestClass.cs (Version1)	Test class providing version identification methods for the 1.0.0 assembly
System.Runtime.Loader.Test.AssemblyVersion3.csproj	Test assembly project configured for version 3.0.0 with shared assembly name
VersionTestClass.cs (Version3)	Test class providing version identification methods for the 3.0.0 assembly