fix: pre-read bodies with unknown length

proxy/capture.go

@@ -13,7 +13,7 @@ import (
 
 // readRequestBody 在转发前完整读取请求体，确保读取失败时不会向上游发送损坏请求。
 func readRequestBody(r *http.Request, max int64) (captured []byte, truncated bool, err error) {
-	if r.Body == nil || r.ContentLength == 0 {
+	if r.Body == nil || r.Body == http.NoBody {
 		return nil, false, nil
 	}
 	body, err := io.ReadAll(r.Body)

tests/proxy/robustness_test.go

@@ -100,6 +100,30 @@ func TestRequestBodyReadFailureReturnsFixed400WithoutUpstream(t *testing.T) {
 	}
 }
 
+func TestRequestBodyReadFailureWithZeroContentLengthDoesNotReachUpstream(t *testing.T) {
+	var upstreamCalls atomic.Int32
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		upstreamCalls.Add(1)
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer upstream.Close()
+	u, _ := url.Parse(upstream.URL)
+	h := newTestHandler(t, u, &captureSubmitter{}, proxy.Options{})
+
+	req := httptest.NewRequest(http.MethodPost, "http://proxy.test/v1/chat", nil)
+	req.Body = failingBody{err: errors.New("secret read failure")}
+	req.ContentLength = 0
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusBadRequest || rec.Body.String() != "bad request\n" {
+		t.Fatalf("response=(%d, %q), want fixed 400 bad request", rec.Code, rec.Body.String())
+	}
+	if upstreamCalls.Load() != 0 {
+		t.Fatalf("upstream called %d times", upstreamCalls.Load())
+	}
+}
+
 func TestRequestBodyReadFailureAfterCaptureLimitDoesNotReachUpstream(t *testing.T) {
 	var upstreamCalls atomic.Int32
 	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {