gateway: a reply translated from a dying stream no longer reads complete - #78

Root cause

Server.translate speaks one provider API to the client while streaming another from the provider, reading the provider's SSE stream with readSSE (internal/gateway/sse.go). readSSE returns the stream's terminal error, but both call sites in translate ignore that return (internal/gateway/gateway.go).

So when a provider's connection dies mid-reply — upstream reset, truncated body — the failure vanishes:

• A buffered client (stream:false) gets HTTP 200 with whatever partial text had arrived, rendered with finish_reason: "stop" — a fragment indistinguishable from a complete answer.
• A streaming client gets a synthetic happy ending: a stop chunk, final usage, and [DONE] over a stream the provider never finished.

Repro on current main (f298bb4): an httptest upstream that hijacks the connection, promises 100 more bytes than it sends, delivers one message_start and one text delta, then closes. Buffered: 200 with "content":"hel" and finish_reason:"stop". Streamed: a finish_reason:"stop" chunk, usage, [DONE], and no error anywhere.

Fix

Check readSSE's return in both branches:

• Buffered: a stream error now fails the request with 502 naming the provider (UP: unexpected EOF) instead of returning the partial answer. A partial answer is not an answer.
• Streamed: the 200 is already committed, so translate emits the client protocol's own error event — {"error": ...} for Chat, response.failed for Responses, event: error for Anthropic, an error chunk for Gemini, each encoder's existing KError rendering — and skips the normal completion instead of appending a synthetic one. A stream whose provider already signalled an error likewise no longer gets a completion event after it.

Complete streams are untouched: a control test runs a full Anthropic stream through both branches and asserts the same 200, translated body and [DONE] as before.

Tests

• New internal/gateway/translate_severed_test.go:
  • TestTranslateBufferedSeveredUpstream — fails on main (200 with partial body), passes with the fix (502 naming the provider).
  • TestTranslateStreamSeveredUpstream — fails on main (synthetic stop + [DONE]), passes with the fix (error event, no completion).
  • TestTranslateStreamCompleteUpstream — control: a properly finished stream still translates end to end, streamed and buffered.
• go vet ./internal/gateway/ clean; go test -race -count=1 ./internal/gateway/ passes; go test ./internal/... passes except internal/gui, which fails to compile in this Linux container for want of GTK4/WebKitGTK 6 development libraries — pre-existing, since the unmodified tree fails the same way, and CI installs GTK with its own tags.