feat: forward upstream response headers through protocol Response

[lmvdz]

Summary

The LLM client currently discards all upstream HTTP response headers before they reach the Switchyard response builder. Any headers set by the upstream backend (or a proxy in front of it) — such as routing decisions, request IDs, or observability metadata — are lost.

This PR adds an upstream_headers: http::HeaderMap field to protocol::Response and preserves upstream headers through the full client → protocol → server pipeline.

Changes

• protocol: Added upstream_headers: HeaderMap to Response struct
• libsy-llm-client: Captures headers in send_once() before the response body is consumed; passes through EncodedResponse → call_rewrite_model → protocol::Response
• switchyard-server: Forwards non-reserved upstream headers to the downstream client after Switchyard's own routing headers
• Updated all downstream Response constructors across libsy, switchyard-py, usage_metrics, and tests

Forwarding rules (in switchyard-server)

• Excluded: content-type, content-length, transfer-encoding, connection, date, server, x-request-id
• Excluded: x-switchyard-* headers (set by Switchyard itself)
• All other upstream response headers are passed through

Breaking changes

None. The new field defaults to HeaderMap::new() in all constructors.