Implement OpenAPI parameter handling

Overview

Build models and converters for handling parameters and request bodies. This covers all input validation and parameter definitions from Grape params blocks.

(Potential) Acceptance criteria

• Parameter model represents path, query, header, and cookie parameters
• RequestBody model handles body parameters for POST/PUT/PATCH
• ParameterConverter transforms Grape params to Parameter models
• RequestBodyConverter handles body parameters and file uploads
• Required/optional parameters are properly marked
• Parameter types and formats are correctly mapped
• Validations (enum, pattern, min/max, oneOf) are extracted
• Nested parameters are handled for request bodies
• Array parameters are properly represented
• File upload parameters generate appropriate content types (do we use this?)