Phase 2, Task 7: Refactor process_response_streaming to accept W: Write

[aram356]

Parent: #563
Phase: #567

Summary

Change process_response_streaming from returning Body (with internal Vec<u8>) to writing into a generic W: Write parameter. This enables the streaming path to pass StreamingBody directly.

Changes

• Change signature: fn process_response_streaming<W: Write>(body: Body, output: &mut W, params: &ProcessResponseParams) -> Result<(), ...>
• Remove internal let mut output = Vec::new() and Ok(Body::from(output))
• Update call site in handle_publisher_request to pass &mut Vec::new() (preserves buffered behavior)

File: crates/trusted-server-core/src/publisher.rs:97-180

Plan

See docs/superpowers/plans/2026-03-25-streaming-response.md — Task 7