Provide a mechanism to override `classifyResponseStreaming` behaviour

### Problem Statement

I have a React RSC application that is streaming responses, and `classifyResponseStreaming` does not correctly detect the HTTP response as a stream because it is limited to only a few content types. I would like to be able to override the detection behaviour so I can avoid spans ending early.

https://github.com/getsentry/sentry-javascript/blob/develop/packages/cloudflare/src/request.ts#L146

### Solution Brainstorm

It would be ideal if the Sentry SDK had an optional callback that could be set to inspect a response and return whether or not the response is streaming.

### Additional Context

_No response_

### Priority

_No response_

Start in packages/cloudflare/src/request.ts at classifyResponseStreaming and trace its callers to understand how the current content-type checks affect span completion. Define how an optional response-inspection callback should override the built-in classification, then verify that custom streaming responses keep their spans open until streaming finishes.

Written by the indexing model from the issue text.