Phase 1, Task 2: Convert process_gzip_to_gzip to chunk-based processing

[aram356]

Parent: #563
Phase: #566
Depends on: Task 1 (encoder finalization fix)

Summary

process_gzip_to_gzip calls read_to_end() to decompress the entire body into memory. Convert to use process_through_compression for chunk-based processing, matching the deflate and brotli paths.

Changes

• Replace read_to_end() + single process_chunk call with process_through_compression pattern
• Caller calls encoder.finish() explicitly (enabled by Task 1 &mut W signature)
• Add gzip round-trip correctness test

File: crates/trusted-server-core/src/streaming_processor.rs:183-225

Plan

See docs/superpowers/plans/2026-03-25-streaming-response.md — Task 2