Added optional support for callbacks on streamed response #322

[abhoy21]

Related to #322

This PR implements callbacks for streamed responses to improve developer experience. Developers can now easily manage streamed chat responses with callbacks for data chunks, completion, and error handling.

Key changes:

• Added onData, onEnd, and onError callback options to streamed response methods
• Updated type definitions in requests.ts to support the new callback interface
• Modified stream-reader.ts to properly handle the callback pipeline
• Updated relevant model implementations to support the callback pattern

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.