Phase 4, Task 17: Binary pass-through tests and verification

[aram356]

Parent: #563
Depends on: Task 16

Summary

Add tests and run full verification for binary pass-through streaming.

Tests

• Publisher-level test: image content type returns `PassThrough` (not `Buffered`)
• Publisher-level test: 4xx image stays `Buffered`
• Verify `Content-Length` is preserved for pass-through (no chunked encoding needed since body is unmodified)
• Full verification: `cargo test`, clippy, fmt, WASM build

Plan

See `docs/superpowers/specs/2026-03-25-streaming-response-design.md`