Preserve Hub binding for streaming responses in sentry-tower/axum

When using the `sentry-tower`/`axum` integration with streaming responses, the middleware binds the hub only for the request handler. Once the handler returns a streaming body, the middleware unwinds and `axum` drains the stream outside that hub. This causes missing context and any newly set context to be attached to the wrong hub during stream evaluation.

Track the tracing/transaction behavior separately in getsentry/sentry-rust#1053.

A possible direction is to bind the hub to the response stream, similar to getsentry/objectstore#526.

Start by locating the sentry-tower/axum middleware and how it handles the response body after the request handler returns. Compare the response-stream hub-binding approach in getsentry/objectstore#526. Done means streamed response evaluation preserves the request hub and newly set context is attached to that hub; keep tracing and transaction behavior separate per getsentry/sentry-rust#1053.

Written by the indexing model from the issue text.