refactor(openapi): extract operation schema building into OperationSchemaBuilder

[eadwinCode]

Introduce Builder pattern for OpenAPI operation schema generation by
extracting operation-specific logic from OpenAPISchema into a dedicated
OperationSchemaBuilder class.

Changes:

• Add new OperationSchemaBuilder class with OperationSchemaResult dataclass
• Move parameter, request body, response, and security schema building
  to the OperationSchemaBuilder
• Simplify OpenAPISchema to focus on document computation
• Add get_operation_builder_type() method to Operation for extensibility
• Export new classes from ninja.openapi module