Refactor stream handling to expose event labels

[ingemar]

Description:

This PR refactors how streaming responses are handled to make stream event labels accessible to the user-defined stream handler.

Why This Change?

This update enables access to the event labels in streamed OpenAI API responses. These labels provide important context (e.g., response.created, response.completed) previously unavailable to user-defined stream handlers.

Exposing this metadata makes building more responsive and structured streaming experiences easier, while keeping the API flexible and backward compatible.

Summary of Changes

• Moved the code from OpenAI::HTTP#to_json_stream into a new class named OpenAI::Stream
• OpenAI::Stream maintains backward compatibility by inspecting the arity of the proc and calling it with one or two arguments accordingly
• Replaced all calls to the #to_json_stream method with the new OpenAI::Stream interface
• Updated all examples and specs to replace unused _bytesize params with _event (since, as far as I understand, the bytesize was never actually passed)

Example Before vs. After

Before:

stream: proc do |chunk, _bytesize|
  puts chunk["delta"]
end

After:

stream: proc do |chunk, event|
  puts "[#{event}] #{chunk["delta"]}"
end

Compatibility

• Fully backward compatible: if a stream proc only takes one argument, it will behave as before
• All tests passing

---

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do and why you'd like us to include them?

[ingemar]

We need to convert to a Proc because Faraday enforces the stream handler to be that.

[ingemar]

This could be changed to check the event instead. Maybe that would be more elegant.
Example:

next if event == "done"