Preserve pre-compressed upstream responses #113

Description

Offer an explicit pass-through mode that retains an upstream response's encoded bytes instead of always decoding and later recompressing them.

Fastly and Cloudflare proxy conversion currently feeds upstream streams through decode transforms; Spin similarly decompresses supported encodings. Main exposes no portable pass-through selection on the single-request proxy interface.

Steps to reproduce

1. Inspect upstream response conversion in the Fastly/Cloudflare proxy clients.
2. Compare the proxy request/options surface with the required encoded pass-through mode.

Expected behavior

Applications can deliberately preserve a compatible upstream representation with its correct encoding metadata.

Impact

A proxy may spend CPU decoding/re-encoding content and cannot consistently preserve an origin representation across providers.

Proposed fix

Define a portable decode-versus-preserve policy and implement it across proxy clients, accounting for provider-managed decoding and client encoding compatibility. Coordinate with #275 and #452.

Done when

• Pass-through preserves supported encoded bytes and matching Content-Encoding metadata.
• Decode mode still removes/recomputes representation headers correctly.
• No double compression occurs and incompatible client encodings have defined behavior.
• Tests exercise encoded upstream responses on each supported adapter and document provider constraints.

Related

• Parent: Complete streaming and compression across adapters #107.
• Automatic outgoing compression: Automatically compress responses from Accept-Encoding #111.
• Fastly upstream decode path
• Cloudflare upstream decode path
• Current single-request outbound interface
• Implementation PR feat: implement portable outbound HTTP and request lifecycle contracts #275 supplies explicit ResponseDecoding::Original, shared encoded-body/header preservation, all-adapter wiring and controlled local relay coverage. Keep this issue open until its complete scope is accepted and merged; PR Add explicit adapter delivery compression support #452 remains related delivery-compression work.
• Related PR Add explicit adapter delivery compression support #452; this PR alone does not establish completion of this issue.