Prevent migration code lens from failing in unsaved files

[vinistock]

Our code lens code assumes that migration files are already saved to disk, by running File.basename(T.must(@path)).

It seemed like a reasonable assumption, but judging by our error telemetry, it seems people will write unsaved migrations more often that I expected. We can't run a migration that wasn't saved yet, so we just need to ensure the code doesn't break.

Note: there's currently no way to test this because the with_server test helper assumes that the URI passed will exist on disk, which is a limitation of our helper. This means I can't pass an untitled:Untitled-1 URI to reproduce the issue.

You can verify that the issue is fixed after this change by pasting this migration into an unsaved file.

class CreateUsers < ActiveRecord::Migration[7.0]
  def change
    create_table(:users) do |t|
      t.string(:first_name)
      t.string(:last_name)
      t.integer(:age)

      t.timestamps
    end
  end
end