Axum: preserve incremental streaming with bounded backpressure #394

Description

Main collects Body::Stream into an unbounded Vec before returning an Axum response; crates/edgezero-adapter-axum/src/response.rs:19. Proxy uploads/downloads also collect entire bodies. PR #275 already replaces these seams with owned HTTP/1 egress and portable streamed outbound contracts. This issue tracks residual production streaming acceptance after integration.

Steps to reproduce

Use delayed multi-chunk and open-ended responses, a slow/zero-read client, large uploads/downloads, a disconnect, a failing producer and shutdown. Inspect first-byte timing, retained bytes/chunks and exactly-once cleanup on the actual native transport.

Expected behavior

Clients receive chunks before producer EOF; producer progress and retained body data follow documented transport bounds. Disconnect/error/deadline/shutdown paths release owned work without hangs.

Impact

Whole-body collection delays first byte, makes open-ended responses unusable and scales retained memory with transfer size. Finite in-memory fixtures do not prove backpressure.

Proposed fix

Adopt accepted #275 streaming, connection and response-egress contracts; reuse its delayed/error/disconnect tests and add only missing production cases. Coordinate #392 draining and #395 proxy acceptance. Preserve non-Send core bodies and BestEffort completion/backpressure/abort/write-deadline classifications; server terminal reports do not prove end-client receipt. Do not add a second channel bridge or arbitrary stream-lifetime limits.

Done when

• Re-audit the accepted PR feat: implement portable outbound HTTP and request lifecycle contracts #275 implementation and record which checks are already covered. Add only residual changes and missing production acceptance evidence.
• A delayed multi-chunk response reaches the client before the producer emits its final chunk.
• An SSE-style or open-ended stream remains usable without waiting for end-of-stream.
• A slow consumer keeps producer progress/retained body data within the documented transport bound. Record native runtime evidence without asserting end-client receipt.
• Large uploads and proxied downloads have bounded retained body data and preserve bytes/status/headers.
• Consumer disconnect, source error, deadline, and shutdown clean up producer work without hangs.
• Tests use delayed and failing streams, not just ready-in-memory fixtures.
• Existing WASM target checks and portable core tests still pass.

Related

• Parent Make the Axum adapter production-ready for self-hosted deployments #391. Foundation PR feat: implement portable outbound HTTP and request lifecycle contracts #275 / Implement portable outbound HTTP and request lifecycle contracts #436 is related implementation, not automatic completion of this acceptance issue.
• Complete streaming and compression across adapters #107/Provide a core streaming response helper #110 portable streaming; Axum: define a production server lifecycle with bounded graceful shutdown #392 process draining; Axum: reuse outbound clients and preserve proxy HTTP semantics #395 outbound semantics.
• No separate published PR currently covers all residual Axum: preserve incremental streaming with bounded backpressure #394 acceptance.