fix: 压缩RDP实时画面传输

- 将原始RGBA位图压缩为PNG后通过WebSocket发送
- 降低低带宽代理链路下的RDP首帧延迟
- 增加位图压缩与异常数据测试

backend/controller/rdp_live_frame_test.go

@@ -0,0 +1,66 @@
+package controller
+
+import (
+	"bytes"
+	"encoding/base64"
+	"image/png"
+	"strings"
+	"testing"
+
+	rdpengine "ezops/pkg/rdp"
+)
+
+func TestEncodeRDPBitmapPNGCompressesFrame(t *testing.T) {
+	const width = 900
+	const height = 540
+	rgba := make([]byte, width*height*4)
+	for i := 0; i < len(rgba); i += 4 {
+		rgba[i+3] = 255
+	}
+
+	encoded, err := encodeRDPBitmapPNG(rdpengine.ServerFrame{
+		Type:   "bitmap",
+		X:      12,
+		Y:      34,
+		Width:  width,
+		Height: height,
+		RGBA:   rgba,
+	})
+	if err != nil {
+		t.Fatalf("压缩 RDP 位图失败: %v", err)
+	}
+	if len(encoded.RGBA) != 0 {
+		t.Fatalf("压缩后仍携带原始 RGBA: %d", len(encoded.RGBA))
+	}
+	const prefix = "data:image/png;base64,"
+	if !strings.HasPrefix(encoded.Image, prefix) {
+		t.Fatalf("压缩帧格式错误: %q", encoded.Image[:min(len(encoded.Image), len(prefix))])
+	}
+	pngData, err := base64.StdEncoding.DecodeString(strings.TrimPrefix(encoded.Image, prefix))
+	if err != nil {
+		t.Fatalf("解码 PNG 数据失败: %v", err)
+	}
+	img, err := png.Decode(bytes.NewReader(pngData))
+	if err != nil {
+		t.Fatalf("解析 PNG 数据失败: %v", err)
+	}
+	if img.Bounds().Dx() != width || img.Bounds().Dy() != height {
+		t.Fatalf("PNG 尺寸错误: got=%dx%d", img.Bounds().Dx(), img.Bounds().Dy())
+	}
+	if len(encoded.Image) >= len(rgba)/4 {
+		t.Fatalf("PNG 压缩率不足: encoded=%d raw=%d", len(encoded.Image), len(rgba))
+	}
+	t.Logf("RDP 帧压缩: raw=%d encoded=%d", len(rgba), len(encoded.Image))
+}
+
+func TestEncodeRDPBitmapPNGRejectsShortPayload(t *testing.T) {
+	_, err := encodeRDPBitmapPNG(rdpengine.ServerFrame{
+		Type:   "bitmap",
+		Width:  10,
+		Height: 10,
+		RGBA:   make([]byte, 8),
+	})
+	if err == nil {
+		t.Fatal("短位图数据应返回错误")
+	}
+}