Resolve Style/ModuleMemberExistenceCheck (#554)

## What this does

<!-- Clear description of what this PR does and why -->
Noticed that `bundle exec rubocop` was failing in main for me after a
`bundle update`

## Type of change

- [x] Bug fix
- [ ] New feature
- [ ] Breaking change
- [ ] Documentation
- [ ] Performance improvement

## Scope check

- [x] I read the [Contributing
Guide](https://github.com/crmne/ruby_llm/blob/main/CONTRIBUTING.md)
- [x] This aligns with RubyLLM's focus on **LLM communication**
- [x] This isn't application-specific logic that belongs in user code
- [x] This benefits most users, not just my specific use case

## Quality check

- [x] I ran `overcommit --install` and all hooks pass
- [x] I tested my changes thoroughly
- [x] For provider changes: Re-recorded VCR cassettes with `bundle exec
rake vcr:record[provider_name]`
  - [x] All tests pass: `bundle exec rspec`
- [x] I updated documentation if needed
- [x] 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

<!-- Link issues: "Fixes #123" or "Related to #123" -->

Co-authored-by: Carmine Paolino <[email protected]>

Author: tpaulshippy

lib/ruby_llm/tool.rb

@@ -186,7 +186,7 @@ def resolve_schema
       def resolve_direct_schema(schema)
         return extract_schema(schema.to_json_schema) if schema.respond_to?(:to_json_schema)
         return RubyLLM::Utils.deep_dup(schema) if schema.is_a?(Hash)
-        if schema.is_a?(Class) && schema.instance_methods.include?(:to_json_schema)
+        if schema.is_a?(Class) && schema.method_defined?(:to_json_schema)
           return extract_schema(schema.new.to_json_schema)
         end