fix: implement Flush on trackingResponseWriter

Allows the trackingResponseWriter to properly proxy flush
requests to the underlying http.Flusher. This ensures
compatibility with streaming responses.

cmd/api-response_test.go

@@ -24,6 +24,7 @@ import (
 	"testing"
 
 	"github.com/klauspost/compress/gzhttp"
+	xhttp "github.com/minio/minio/internal/http"
 )
 
 // Tests object location.
@@ -159,6 +160,29 @@ func TestTrackingResponseWriter(t *testing.T) {
 	}
 }
 
+func TestTrackingResponseWriterFlush(t *testing.T) {
+	rw := httptest.NewRecorder()
+	trw := &trackingResponseWriter{ResponseWriter: rw}
+
+	trw.Flush()
+	if trw.headerWritten {
+		t.Fatal("Flush() should not set headerWritten")
+	}
+
+	// Simulate the ListenNotificationHandler flow: WriteHeader, Write, Flush
+	trw.WriteHeader(http.StatusOK)
+	_, err := trw.Write([]byte("event data"))
+	if err != nil {
+		t.Fatalf("Write failed: %v", err)
+	}
+
+	xhttp.Flush(trw)
+
+	if !rw.Flushed {
+		t.Fatalf("xhttp.Flush should have flushed the underlying ResponseRecorder via trackingResponseWriter.Flush()")
+	}
+}
+
 func TestHeadersAlreadyWritten(t *testing.T) {
 	rw := httptest.NewRecorder()
 	trw := &trackingResponseWriter{ResponseWriter: rw}