fix(gateway): CC 桥接受 Anthropic 上游的 SSE 紧凑格式

SSE 规范允许字段冒号后省略空格（event:message_start / data:{...}）。
Kimi 等 Anthropic 兼容上游返回紧凑格式时，Anthropic → Chat Completions
桥按 "event: " / "data: " 严格匹配会丢弃全部事件，且 dataLine[6:] 按带
空格前缀硬切载荷，最终流式/缓冲两条路径都报 "Upstream stream ended
without a response"（与 #4653 同根因；#4657 只修了 /v1/responses 桥，
本补丁补齐对称的 /v1/chat/completions 桥）。

复用 OpenAI 链路既有的 extractOpenAISSEEventLine / extractOpenAISSEDataLine
（正确处理可选空格/tab），流式与缓冲两处统一替换；补充紧凑格式的
缓冲/流式回归测试，原带空格格式用例保持通过。

backend/internal/service/gateway_forward_as_chat_completions.go

@@ -240,18 +240,19 @@ func (s *GatewayService) handleCCBufferedFromAnthropic(
 
 	for scanner.Scan() {
 		line := scanner.Text()
-		if !strings.HasPrefix(line, "event: ") {
+		// SSE 规范允许 `event:xxx`（冒号后无空格）：Kimi 等 Anthropic 兼容上游
+		// 返回紧凑格式，严格匹配 "event: " 会丢弃全部事件（#4653 同根因）。
+		if _, ok := extractOpenAISSEEventLine(line); !ok {
 			continue
 		}
 
 		if !scanner.Scan() {
 			break
 		}
-		dataLine := scanner.Text()
-		if !strings.HasPrefix(dataLine, "data: ") {
+		payload, ok := extractOpenAISSEDataLine(scanner.Text())
+		if !ok {
 			continue
 		}
-		payload := dataLine[6:]
 
 		var event apicompat.AnthropicStreamEvent
 		if err := json.Unmarshal([]byte(payload), &event); err != nil {
@@ -447,18 +448,18 @@ func (s *GatewayService) handleCCStreamingFromAnthropic(
 
 	for scanner.Scan() {
 		line := scanner.Text()
-		if !strings.HasPrefix(line, "event: ") {
+		// 与缓冲路径一致：接受 SSE 紧凑格式（冒号后无空格，#4653 同根因）。
+		if _, ok := extractOpenAISSEEventLine(line); !ok {
 			continue
 		}
 
 		if !scanner.Scan() {
 			break
 		}
-		dataLine := scanner.Text()
-		if !strings.HasPrefix(dataLine, "data: ") {
+		payload, ok := extractOpenAISSEDataLine(scanner.Text())
+		if !ok {
 			continue
 		}
-		payload := dataLine[6:]
 
 		var event apicompat.AnthropicStreamEvent
 		if err := json.Unmarshal([]byte(payload), &event); err != nil {

backend/internal/service/gateway_forward_as_chat_completions_test.go

@@ -75,6 +75,78 @@ func TestHandleCCBufferedFromAnthropic_PreservesMessageStartCacheUsageAndReasoni
 	require.Equal(t, "high", *result.ReasoningEffort)
 }
 
+// Kimi 等 Anthropic 兼容上游返回 SSE 紧凑格式（冒号后无空格），CC 桥此前按
+// "event: " / "data: " 严格匹配会丢弃全部事件，最终报 "Upstream stream ended
+// without a response"（#4653 同根因；#4657 只修了 /v1/responses 桥）。
+func TestHandleCCBufferedFromAnthropic_CompactSSEFormat(t *testing.T) {
+	t.Parallel()
+	gin.SetMode(gin.TestMode)
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+
+	resp := &http.Response{
+		Header: http.Header{"x-request-id": []string{"rid_cc_buffered_compact"}},
+		Body: io.NopCloser(strings.NewReader(strings.Join([]string{
+			`event:message_start`,
+			`data:{"type":"message_start","message":{"id":"msg_c1","type":"message","role":"assistant","content":[],"model":"k3","stop_reason":"","usage":{"input_tokens":15,"cache_read_input_tokens":5,"cache_creation_input_tokens":2}}}`,
+			``,
+			`event:content_block_start`,
+			`data:{"type":"content_block_start","index":0,"content_block":{"type":"text","text":"OK"}}`,
+			``,
+			`event:message_delta`,
+			`data:{"type":"message_delta","delta":{"stop_reason":"end_turn"},"usage":{"output_tokens":3}}`,
+			``,
+		}, "\n"))),
+	}
+
+	svc := &GatewayService{}
+	result, err := svc.handleCCBufferedFromAnthropic(resp, c, "k3", "k3", nil, time.Now())
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	require.Equal(t, 15, result.Usage.InputTokens)
+	require.Equal(t, 3, result.Usage.OutputTokens)
+	require.Equal(t, 5, result.Usage.CacheReadInputTokens)
+	require.Equal(t, 2, result.Usage.CacheCreationInputTokens)
+	require.Contains(t, rec.Body.String(), `"OK"`, "紧凑格式事件必须被解析并产出响应内容")
+}
+
+func TestHandleCCStreamingFromAnthropic_CompactSSEFormat(t *testing.T) {
+	t.Parallel()
+	gin.SetMode(gin.TestMode)
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+
+	resp := &http.Response{
+		Header: http.Header{"x-request-id": []string{"rid_cc_stream_compact"}},
+		Body: io.NopCloser(strings.NewReader(strings.Join([]string{
+			`event:message_start`,
+			`data:{"type":"message_start","message":{"id":"msg_c2","type":"message","role":"assistant","content":[],"model":"k3","stop_reason":"","usage":{"input_tokens":21,"cache_read_input_tokens":6,"cache_creation_input_tokens":1}}}`,
+			``,
+			`event:content_block_start`,
+			`data:{"type":"content_block_start","index":0,"content_block":{"type":"text","text":"OK"}}`,
+			``,
+			`event:message_delta`,
+			`data:{"type":"message_delta","delta":{"stop_reason":"end_turn"},"usage":{"output_tokens":4}}`,
+			``,
+			`event:message_stop`,
+			`data:{"type":"message_stop"}`,
+			``,
+		}, "\n"))),
+	}
+
+	svc := &GatewayService{}
+	result, err := svc.handleCCStreamingFromAnthropic(resp, c, "k3", "k3", nil, time.Now(), true)
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	require.Equal(t, 21, result.Usage.InputTokens)
+	require.Equal(t, 4, result.Usage.OutputTokens)
+	require.Equal(t, 6, result.Usage.CacheReadInputTokens)
+	require.Equal(t, 1, result.Usage.CacheCreationInputTokens)
+	require.Contains(t, rec.Body.String(), `[DONE]`)
+}
+
 func TestHandleCCStreamingFromAnthropic_PreservesMessageStartCacheUsageAndReasoning(t *testing.T) {
 	t.Parallel()
 	gin.SetMode(gin.TestMode)