fix: 彻底解决UDP插件阻塞导致扫描无法结束的问题

根因分析（通过 goroutine dump 定位）:
1. UDP conn.Write 在 WSL2 上可能永久阻塞（SetDeadline 对 Write 不生效）
2. SNMP community 爆破混入通用密码字典（57个），串行 × 10s超时 = 10分钟

修复:
- 提取 udpProbe() 公共函数，用 context timeout + conn.Close 双保险
  超时后强制关闭连接，中断阻塞的 Write/Read
- BACnet/DNS/IPMI/TFTP 统一使用 udpProbe()
- SNMP probe 使用 goroutine + context select 保护
- SNMP community 列表不再混入通用密码字典（8个专用 community 足够）
- SNMP 后续 community 爆破用 3s 短超时 + 连续失败 3 次快速退出

效果: 同样的扫描从无限卡死 → 9秒完成

plugins/services/udp_parsers_test.go

@@ -95,13 +95,12 @@ func TestSNMPBuildersAndCommunityList(t *testing.T) {
 	}
 
 	cfg := common.NewConfig()
-	cfg.Credentials.Passwords = []string{"private", "custom", "public"}
 	communities := NewSNMPPlugin().buildCommunityList(cfg)
-	if !containsString(communities, "public") || !containsString(communities, "private") || !containsString(communities, "custom") {
+	if !containsString(communities, "public") || !containsString(communities, "private") {
 		t.Fatalf("community list missing expected entries: %v", communities)
 	}
-	if countString(communities, "public") != 1 || countString(communities, "private") != 1 {
-		t.Fatalf("community list should deduplicate entries: %v", communities)
+	if len(communities) != 8 {
+		t.Fatalf("community list should have 8 entries, got %d: %v", len(communities), communities)
 	}
 }