enhancement: update streaming code to follow `EventSource` protocol

[ajndkr]

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🔖 Feature description

The project uses lanarky for langchain streaming. After v0.8,

• EventSource protocol is enforced for server-sent events
• StreamingResponse for langchain objects has been refactored

Related documentation: https://lanarky.ajndkr.com/learn/adapters/langchain/fastapi/#streaming

✔️ Solution

As the author of lanarky, I would love to assist here. I can handle the python side but someone else needs to take over the client side changes.

❓ Alternatives

No response

📝 Additional Context

No response

Acknowledgements

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[ajndkr]

cc @awtkns