Use @discardableResult consistently on completion-handler methods #466

Completion-handler methods return a CancellableRequest, which callers often don't need. Whether discarding it produces an "unused result" warning currently depends on the method:

Add @discardableResult to every completion-handler method that returns a CancellableRequest, in the protocols and in the concrete implementations (OpenAI, ResponsesEndpoint). Callers that use the concrete types directly get the same behaviour that way.

Adding @discardableResult is source-compatible: it only removes a warning. Run swift package diagnose-api-breaking-changes to confirm it reports nothing.