Gracefully degrade when the source generated document isn't available

[davidwengier]

We should log an error, but generally speaking we should make sure to light up as much Razor and Html tooling features as we can.

[davidwengier]

Further to the above, we should introduce a base class for all our endpoints that checks context.TextDocument for null, and logs a message (once per document/folder/project?) with a warning about needing to use additional files.

See https://github.com/dotnet/razor/pull/11959/files#r2153653867 though, and make sure we don't log messages that don't make sense (ie, in .NET Framework projects)