fix: remove empty Dispose and fix nullability annotation

[thomhurst]

Summary

Closes#4858

• Remove unnecessary IDisposable implementation from EventReceiverOrchestrator: The Dispose() method was empty (the underlying EventReceiverRegistry no longer uses ReaderWriterLockSlim). No callers explicitly call Dispose() on this type -- it was only invoked by TUnitServiceProvider.DisposeAsync() which iterates registered services and disposes any that implement IDisposable. Removing the interface means the service provider simply skips it during cleanup.
• Add nullable annotation to PropertyInjector.InjectPropertiesRecursiveAsync: The instance parameter was typed as non-nullable object but had a null guard that returned early. This is correct behavior for a recursive method (nested property values can be null), so the parameter is now annotated as object? to match the actual contract.

Test plan

• dotnet build TUnit.Engine/TUnit.Engine.csproj succeeds with 0 warnings and 0 errors across all target frameworks (netstandard2.0, net8.0, net9.0, net10.0)
• CI passes

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

Both changes are well-scoped and correct:

EventReceiverOrchestrator: Remove IDisposable
The empty Dispose() removal is safe — EventReceiverRegistry no longer holds a ReaderWriterLockSlim (or any other disposable resource), so there is nothing to clean up. TUnitServiceProvider.DisposeAsync() does a runtime is IDisposable check before calling Dispose(), so skipping this class during teardown is the right outcome.

PropertyInjector: object → object?
The nullability annotation now matches the method's long-standing behavior. The null guard at the top of InjectPropertiesRecursiveAsync already handled null inputs; annotating the parameter accordingly eliminates a misleading non-nullable contract on what is effectively a nullable-accepting recursive helper. The public InjectPropertiesAsync entry point still enforces non-null via ArgumentNullException, so the public API contract is unchanged.