Check for enums not using smallint

What does this MR do?

This MR adds a schema test to check that any new enum introduced has the type of smallint. In schema.rb, this is indicated by limit: 2 in the column definition.

Closes #25012 (closed)

Screenshots

Example spec failure:

Failures:

  1) Database schema for enums User uses smallint for enums
     Failure/Error: expect(model).to use_smallint_for_enums(enums)
     
       Expected User enums: layout, dashboard, project_view to use the smallint type.

       The smallint type is 2 bytes which is more than sufficient for an enum.
       Using the smallint type would help us save space in the database.
       To fix this, please add `limit: 2` in the migration file, for example:
     
       def change
         add_column :ci_job_artifacts, :file_format, :integer, limit: 2
       end
     # ./spec/db/schema_spec.rb:157:in `block (5 levels) in <top (required)>'

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers