fix(stream): report aborts after HTTP 200 in-band instead of closing silently

A stream that stalled or lost its upstream was closed with no terminal frame
at all, so clients saw "200 OK, a few chunks, then nothing" and could not tell
a truncated reply from a finished one. The Responses passthrough path already
synthesized response.failed; every other client format got nothing.

The watchdog now hands its reason ("stream stall timeout" or "upstream
connection lost") to onAbortTerminal, and buildStreamErrorBytes frames it per
client format: OpenAI-compatible clients get data: {"error":{...}} followed by
data: [DONE], Anthropic clients get `event: error`. The error frame always
precedes [DONE] (openai-python raises APIError on any data payload carrying an
error key), and no synthetic finish_reason is ever emitted — a truncated
stream must not look like a clean stop.

Co-Authored-By: Claude Code <noreply@anthropic.com>

open-sse/utils/streamHelpers.js

@@ -1,4 +1,8 @@
 import { FORMATS } from "../translator/formats.js";
+import { buildErrorBody } from "./error.js";
+import { SSE_DONE } from "./sseConstants.js";
+
+const sharedEncoder = new TextEncoder();
 
 // Parse SSE data line
 export function parseSSELine(line, format = null) {
@@ -120,3 +124,24 @@ export function formatSSE(data, sourceFormat) {
 
   return `data: ${JSON.stringify(data)}\n\n`;
 }
+
+// Terminal frames for a stream that aborted after HTTP 200 was already sent, so
+// the status code can no longer change. OpenAI-compatible clients (openai-python
+// raises APIError on any `data:` payload carrying an `error` key, checked before
+// [DONE]) need the error frame first, then [DONE]; Anthropic clients need
+// `event: error`. Never fabricate a successful finish_reason instead.
+//
+// Returns encoded bytes: onAbortTerminal callbacks are enqueued verbatim, same
+// as buildAbortedResponsesTerminalBytes.
+//
+// NOTE: non-SSE client formats (Ollama NDJSON) get an SSE frame here — dead in
+// practice because detectFormatByEndpoint never resolves to OLLAMA.
+export function buildStreamErrorBytes(statusCode, message, clientFormat) {
+  const { error } = buildErrorBody(statusCode, message);
+
+  const sse = clientFormat === FORMATS.CLAUDE
+    ? formatSSE({ type: "error", error }, FORMATS.CLAUDE)
+    : formatSSE({ error }, clientFormat) + SSE_DONE;
+
+  return sharedEncoder.encode(sse);
+}