fix: replace Axios responseType stream with native fetch for browser compatibility

[Senthil455]

Problem

copilotApi.chat.stream() used api.post(..., { responseType: "stream" }) which is an Axios feature only available in Node.js. In browser environments, Axios does not support responseType: "stream" — the call would silently fail or throw.

Solution

Replaced the Axios call with native fetch() that returns a Promise. The response body provides a ReadableStream via response.body.getReader() which is the standard browser API for consuming streaming responses.

The Authorization header is set from the in-memory token (via getAccessToken()), consistent with the rest of the API layer.

[Senthil455]

gh: Not Found (HTTP 404)
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