Change IRemoteServiceInvoker.TryInvokeAsync(...) to all callers to handle failures gracefully

[DustinCampbell]

Currently, the IRemoteServiceInvoker is defined like so:

internal interface IRemoteServiceInvoker
{
    ValueTask<TResult?> TryInvokeAsync<TService, TResult>(
        Solution solution,
        Func<TService, RazorPinnedSolutionInfoWrapper, CancellationToken, ValueTask<TResult>> invocation,
        CancellationToken cancellationToken,
        [CallerFilePath] string? callerFilePath = null,
        [CallerMemberName] string? callerMemberName = null)
        where TService : class;
}

By returning TResult?, it's impossible for a caller to tell the difference between null as a valid result and null because the remote service call failed. This issue tracks changing this method to return Optional<TResult> and updating callers to handle the result gracefully.

Alternatively, the implementation of IRemoteServiceInvoker could be changed to throw on failures rather than returning default.