Preserve IPv6 wildcard binds in preconn-managed services

[Copilot]

tcpPreconn was rewriting explicit dual-stack listeners from [::]:port to 0.0.0.0:port, which dropped IPv6 ingress for tunnel and forward services using the default listen address. The preconn address parser also had no focused regression coverage for wildcard and IPv6 bind cases.

• Problem

  • Preconn startup normalized :: as IPv4 wildcard instead of preserving the configured bind semantics.
  • Services generated with [::] in the backend became IPv4-only once handed to tcp_pool.

• Code changes

  • Preconn listen parsing
    • Updated go-gost/x/socket/preconn_manager.go so parseListenAddr preserves an explicit IPv6 wildcard host (::).
    • Kept existing fallback behavior for empty host / 0.0.0.0 inputs to avoid changing legacy IPv4-default cases.
  • Regression coverage
    • Added go-gost/x/socket/preconn_manager_test.go.
    • Locked in behavior for:
      • :port
      • 0.0.0.0:port
      • [::]:port
      • [2001:db8::1]:port

• Result

  • Preconn-managed tunnel and forward services now keep the backend’s intended bind address.
  • Default dual-stack listeners remain IPv6-capable instead of being collapsed to IPv4-only.

// before
if host == "" || host == "0.0.0.0" || host == "::" {
	host = "0.0.0.0"
}

// after
if host == "" || host == "0.0.0.0" {
	host = "0.0.0.0"
}