fix(openai): also emit response.failed in ensureForwardErrorResponse after Writer.Written

Case B: when a slot wait flushes SSE ping comments first (Writer.Written
becomes true), the previous ensureForwardErrorResponse short-circuited
on `c.Writer.Written()` and returned false without notifying the client.
Subsequent upstream errors (http2 timeout, stream INTERNAL_ERROR, etc.)
produced silent EOF; Codex CLI reported "stream closed before
response.completed" just like the user-slot timeout case.

Remove the Written() early return; coerce streamStarted to true when
Writer has already been written to, and let handleStreamingAwareError
walk the existing logic — which now (thanks to the previous commits)
emits a protocol-compliant response.failed for /responses paths and the
legacy `event: error` for others.

Update tests that previously asserted "do not override written response":
the new contract is to *append* an SSE terminal frame so the client sees
a clean close instead of EOF. recoverResponsesPanic inherits this fix.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/handler/gateway_handler_error_fallback_test.go

@@ -33,7 +33,9 @@ func TestGatewayEnsureForwardErrorResponse_WritesFallbackWhenNotWritten(t *testi
 	assert.Equal(t, "Upstream request failed", errorObj["message"])
 }
 
-func TestGatewayEnsureForwardErrorResponse_DoesNotOverrideWrittenResponse(t *testing.T) {
+// Writer 已写后 ensureForwardErrorResponse 必须把错误以 SSE 形式追加，
+// 而不是 silent EOF。非 /responses 路径走 legacy data:{"type":"error"} 分支。
+func TestGatewayEnsureForwardErrorResponse_AppendsSSEAfterWritten(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	w := httptest.NewRecorder()
 	c, _ := gin.CreateTestContext(w)
@@ -43,7 +45,27 @@ func TestGatewayEnsureForwardErrorResponse_DoesNotOverrideWrittenResponse(t *tes
 	h := &GatewayHandler{}
 	wrote := h.ensureForwardErrorResponse(c, false)
 
-	require.False(t, wrote)
+	require.True(t, wrote)
 	require.Equal(t, http.StatusTeapot, w.Code)
-	assert.Equal(t, "already written", w.Body.String())
+	assert.Contains(t, w.Body.String(), "already written")
+	assert.Contains(t, w.Body.String(), `data: {"type":"error"`)
+}
+
+// case B 回归：Anthropic-backed /responses，Writer 已被写过时
+// ensureForwardErrorResponse 仍要发 response.failed。
+func TestGatewayEnsureForwardErrorResponse_ResponsesRouteAfterWrittenEmitsResponseFailed(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodPost, EndpointResponses, nil)
+	_, _ = c.Writer.WriteString(":\n\n")
+
+	h := &GatewayHandler{}
+	wrote := h.ensureForwardErrorResponse(c, false)
+
+	require.True(t, wrote)
+	body := w.Body.String()
+	assert.Contains(t, body, ":\n\n")
+	assert.Contains(t, body, "event: response.failed\n")
+	assert.Contains(t, body, `"type":"response.failed"`)
 }