fix: confirm header-only response delivery

proxy/proxy.go

@@ -221,6 +221,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	h.rp.ServeHTTP(cw, r)
 
 	finished := time.Now()
+	cw.ConfirmDelivery()
 	responseComplete := !isStreamResponse(cw.Header()) || cw.SSEComplete()
 	if cw.Complete() && responseComplete && !st.readFailed.Load() && r.Context().Err() == nil {
 		submitOnce.Do(func() { submit(finished) })

proxy/writer.go

@@ -75,6 +75,19 @@ func (c *captureWriter) Flush() {
 	c.flush()
 }
 
+// ConfirmDelivery establishes an observable delivery boundary for responses
+// whose headers were not followed by a body write.
+func (c *captureWriter) ConfirmDelivery() {
+	if c.written != 0 {
+		return
+	}
+	if _, ok := c.ResponseWriter.(http.Flusher); !ok {
+		c.writeFailed = true
+		return
+	}
+	c.flush()
+}
+
 func (c *captureWriter) flush() {
 	if _, ok := c.ResponseWriter.(http.Flusher); !ok {
 		return