Add specs for v1.7/v1.9 upgrade generators

[afurm]

What this does

Adds generator specs for ruby_llm:upgrade_to_v1_7 and ruby_llm:upgrade_to_v1_9 to prevent regressions. Verifies expected migrations are generated, v1.7 updates the initializer + existing acts_as_* declarations when present, and both generators respect custom model mapping args.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
  • Focused tests pass: bundle exec rspec spec/ruby_llm/generators/upgrade_generators_spec.rb
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

Related issues

N/A