Add RuboCop rule to enforce workhorse assisted uploads for content type parameters

Summary

Create a RuboCop static analysis rule to enforce that all REST API endpoints with content type parameters use workhorse assisted uploads.

Proposal

Implement a custom RuboCop cop that:

• Detects REST API endpoint definitions with content type parameters
• Verifies they use workhorse assisted upload patterns
• Flags violations with clear remediation guidance

Related

• Workhorse assisted uploads documentation: https://docs.gitlab.com/development/uploads/#workhorse-assisted-uploads