Remove data migration from CreateModels schema migration

[lbrito1]

The CreateModels schema migration currently has a data migration mixed in:

    say_with_time "Loading models from models.json" do
      RubyLLM.models.load_from_json!
      Model.save_to_database
    end

Having mixed type migrations (data and schema) is usually a code smell. Puritanism aside, it led me to an real world problem: I'm moving from ruby-openai to ruby_llm, and my test suite is failing because test envs typically do db:schema:load, skipping the data migration. So I get RubyLLM::ModelNotFoundError: Unknown model: gpt-4o-mini errors because the table is empty.

[crmne]

Great catch. You were right that this mixed two separate things, so I split them in 97474a9

• create_models is now schema-only.
• Loading models is explicit: rails ruby_llm:load_models (v1.13+).
• Runtime stays DB-first, but if the model table is empty/missing we fall back to models.json (v1.13+), and chats still persist resolved models via find_or_create_by!.

So we keep migrations clean and avoid test/setup breakage (like db:schema:load).