Add fallback behavior for Microsoft.DotNet.MsBuildSdkResolver loading in external API scenarios

[huulinhnguyen-dev]

Fixes #
Fix the issue: Could not load file or assembly Microsoft.DotNet.MsBuildSdkResolver #12578

Context

External API users were experiencing failures when loading Microsoft.DotNet.MSBuildSdkResolver because Assembly.Load with CodeBase was failing. This caused SDK resolution to fail for external tools and applications.

Changes Made

• Modified LoadResolverAssembly in SdkResolverLoader to add fallback behavior for external API users
• External API users now fallback from Assembly.Load to Assembly.LoadFrom when Assembly.Load fails
• Added unit tests to verify both fallback and no-fallback scenarios

Testing

• Added LoadResolverAssembly_MSBuildSdkResolver_ExternalAPIUser_FallbackSucceeds test to verify external API users get successful fallback
• Added LoadResolverAssembly_MSBuildSdkResolver_VisualStudioUser_NoFallback test to verify VS users don't get fallback
• Tests use TestableSdkResolverLoader to control RunningInVisualStudio environment and test path

Notes

This change improves compatibility for external API users while maintaining the existing behavior for Visual Studio users. The fallback ensures that SDK resolution works in more scenarios without breaking VS functionality.