Handle fixtures with no associated model

[vinistock]

Motivation

Some projects use YAML files inside the test/fixtures directory to represent arbitrary data that is not associated to any specific model.

This is a valid use case that Rails allows, but our DSL compiler fails when any fixtures like that are present because it assumes there will be a model.

Implementation

There are essentially two parts to the fix:

1. Started rescuing the fixture formatting error that gets raised when no model is associated with it and we invoke fh.model_class. We don't want generation to stop because we found one of those fixtures
2. In addition to not crashing, we also don't want to generate RBI methods for fixtures that don't have an associated model or that contain invalid formatting. So we need to remove them from the list of methods to be created. I started memoizing method_names so that we can remove invalid fixtures

Tests

Added a test that reproduces the crash.

[paracycle]

Thanks!

[vinistock]

The build failures are unrelated and are happening on main too. I suspect it's the latest Bundler version since we got a lot of new bundler related failures, which we are trying to investigate in #1922.

I'll go ahead and merge this anyway since the errors are unrelated.