Potential future cohost testing enhancements

[davidwengier]

Following on from #10603
We currently have good testing coverage for cohosting endpoints, but there are some rough edges that could be smoothed over, from an authoring point of view, but to also give us a little more coverage over things like serialization.

• Hook up the source generator to actually compile C# from the .razor files in the test project
• Don't require endpoints to expose methods that take Documents for test purposes
• Use the actual language server