fix: session hooks in referenced library projects not executing

[thomhurst]

Summary

Fixes #4583 - [Before(TestSession)] and [After(TestSession)] hooks defined in referenced library projects were not executing.

Problem

The InfrastructureGenerator generates code to load referenced assemblies early (to trigger their module initializers which register hooks). However, the HasPhysicalLocation check was too restrictive:

return correspondingReference is PortableExecutableReference { FilePath: not null and not "" };

This only accepted PortableExecutableReference objects. During IDE design-time builds, project-to-project references are represented as CompilationReference objects. The incremental generator may cache results from these builds, resulting in missing Assembly.Load() calls for library projects.

Solution

• Renamed HasPhysicalLocation to IsLoadableAtRuntime to better reflect its purpose
• Simplified the logic to accept any assembly with a corresponding reference in the compilation
• Any referenced assembly will be available at runtime (either as a compiled DLL or NuGet package)

Test Plan

• Added LibraryTestSessionHooks class to TUnit.TestProject.Library with [Before(TestSession)] and [After(TestSession)] hooks
• Added LibraryTestSessionHookTests in TUnit.TestProject to verify the library's hooks execute
• Verified test passes with dotnet run -- --treenode-filter "/*/*/LibraryTestSessionHookTests/*"