send out the CONNECTION_REFUSED error when refusing a connection (#4250)

So far, we used Connection.destroy, which destroys a connection without
sending out a CONNECTION_CLOSE frame. This is useful (for example) when
receiving a stateless reset, but it's not what we want when the server
refuses an incoming connection. In this case, we want to send out a
packet with a CONNECTION_CLOSE frame to inform the client that the
connection attempt is being rejected.

mock_packet_handler_test.go

@@ -12,6 +12,7 @@ import (
 	reflect "reflect"
 
 	protocol "github.com/quic-go/quic-go/internal/protocol"
+	qerr "github.com/quic-go/quic-go/internal/qerr"
 	gomock "go.uber.org/mock/gomock"
 )
 
@@ -38,6 +39,42 @@ func (m *MockPacketHandler) EXPECT() *MockPacketHandlerMockRecorder {
 	return m.recorder
 }
 
+// closeWithTransportError mocks base method.
+func (m *MockPacketHandler) closeWithTransportError(arg0 qerr.TransportErrorCode) {
+	m.ctrl.T.Helper()
+	m.ctrl.Call(m, "closeWithTransportError", arg0)
+}
+
+// closeWithTransportError indicates an expected call of closeWithTransportError.
+func (mr *MockPacketHandlerMockRecorder) closeWithTransportError(arg0 any) *PacketHandlercloseWithTransportErrorCall {
+	mr.mock.ctrl.T.Helper()
+	call := mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "closeWithTransportError", reflect.TypeOf((*MockPacketHandler)(nil).closeWithTransportError), arg0)
+	return &PacketHandlercloseWithTransportErrorCall{Call: call}
+}
+
+// PacketHandlercloseWithTransportErrorCall wrap *gomock.Call
+type PacketHandlercloseWithTransportErrorCall struct {
+	*gomock.Call
+}
+
+// Return rewrite *gomock.Call.Return
+func (c *PacketHandlercloseWithTransportErrorCall) Return() *PacketHandlercloseWithTransportErrorCall {
+	c.Call = c.Call.Return()
+	return c
+}
+
+// Do rewrite *gomock.Call.Do
+func (c *PacketHandlercloseWithTransportErrorCall) Do(f func(qerr.TransportErrorCode)) *PacketHandlercloseWithTransportErrorCall {
+	c.Call = c.Call.Do(f)
+	return c
+}
+
+// DoAndReturn rewrite *gomock.Call.DoAndReturn
+func (c *PacketHandlercloseWithTransportErrorCall) DoAndReturn(f func(qerr.TransportErrorCode)) *PacketHandlercloseWithTransportErrorCall {
+	c.Call = c.Call.DoAndReturn(f)
+	return c
+}
+
 // destroy mocks base method.
 func (m *MockPacketHandler) destroy(arg0 error) {
 	m.ctrl.T.Helper()