fix(gateway): wrap Anthropic stream EOF as failover error before client output

Anthropic streaming path (gateway_service.go) returned a plain error on
upstream SSE read failure, so the handler-level UpstreamFailoverError check
never fired and the client received a bare `stream_read_error` event,
breaking long-running tasks even when no bytes had been written yet.

The most common trigger is HTTP/2 GOAWAY from api.anthropic.com edge
backends doing graceful rotation: Go's http.Transport surfaces this as
`unexpected EOF` and never auto-retries.

Mirror what the OpenAI and antigravity gateways already do: when the read
error happens before any byte has reached the client (`!c.Writer.Written()`),
return `*UpstreamFailoverError{StatusCode: 502, RetryableOnSameAccount: true}`
so the handler can retry on the same or another account. After client
output has begun, SSE has no resume protocol — keep the existing passthrough
behavior.

Tests cover both branches via streamReadCloser-based fixtures.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/internal/service/gateway_service.go

@@ -7041,6 +7041,20 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 					sendErrorEvent("response_too_large")
 					return &streamingResult{usage: usage, firstTokenMs: firstTokenMs}, ev.err
 				}
+				// 上游中途读错误（unexpected EOF / connection reset 等，常见于 HTTP/2 GOAWAY）：
+				// 若尚未向客户端写过任何字节，包成 UpstreamFailoverError 让 handler 层走 failover/重试。
+				// 已经开始写流时 SSE 协议无 resume，只能透传错误事件给客户端。
+				if !c.Writer.Written() {
+					logger.LegacyPrintf("service.gateway", "Upstream stream read error before any client output (account=%d), failing over: %v", account.ID, ev.err)
+					body, _ := json.Marshal(map[string]string{
+						"error": fmt.Sprintf("upstream stream disconnected: %s", ev.err),
+					})
+					return nil, &UpstreamFailoverError{
+						StatusCode:             http.StatusBadGateway,
+						ResponseBody:           body,
+						RetryableOnSameAccount: true,
+					}
+				}
 				sendErrorEvent("stream_read_error")
 				return &streamingResult{usage: usage, firstTokenMs: firstTokenMs}, fmt.Errorf("stream read error: %w", ev.err)
 			}