Enhance generateContentStream with streamCallbacks support. Fixes #322

[cestercian]

• Adds streamCallbacks as an optional argument to generateContentStream.
• Enables users to receive text responses via callbacks without handling Node.js streams.
• Includes test cases for the new functionality.

Example Usage:

await model.generateContentStream("Tell me a joke", {}, {
   onData: (chunk: string) => process.stdout.write(chunk),  
   onDone: (fullText: string) => console.log("\nFinal output:", fullText),
});

[demoncoder-crypto]

I am working on this issue let me know if its up to expectation

[avinesh2101]

Hey! This change has already been proposed in [Original PR #371]. Submitting duplicate PRs doesn’t add value and can create unnecessary noise for maintainers. It’s best to check existing PRs before submitting new ones. Maybe consider contributing in other meaningful ways!

[cestercian]

Hey @avinesh2101 .Thanks for pointing that out. I didn’t realize this change had already been proposed in PR #371. I’ll make sure to check existing PRs more carefully next time. Appreciate the guidance! I’d love to contribute in other ways—if there are any areas that need help, I’d be happy to take a look.