refactor(channelserver): replace init() handler registration with explicit construction

The handler table was a package-level global populated by init(), making
registration implicit and untestable. Move it to buildHandlerTable()
which returns the map, store it as a Server struct field initialized in
NewServer(), and add a missing-handler guard in handlePacketGroup to log
a warning instead of panicking on unknown opcodes.

server/channelserver/handlers_register_test.go

@@ -168,7 +168,8 @@ func TestHandlerTableRegistered(t *testing.T) {
 	}
 
 	// Verify handler table is populated
-	if len(handlerTable) == 0 {
+	table := buildHandlerTable()
+	if len(table) == 0 {
 		t.Error("handlers table should not be empty")
 	}
 
@@ -181,8 +182,8 @@ func TestHandlerTableRegistered(t *testing.T) {
 	_ = criticalHandlers // We just verify the table is non-empty since handler function names aren't directly accessible
 
 	// Verify minimum handler count
-	if len(handlerTable) < 50 {
-		t.Errorf("handlers count = %d, expected at least 50", len(handlerTable))
+	if len(table) < 50 {
+		t.Errorf("handlers count = %d, expected at least 50", len(table))
 	}
 }
 
@@ -191,8 +192,9 @@ func TestHandlerTableNilSession(t *testing.T) {
 	// but doesn't call handlers (which would require a real session)
 	_ = createMockServer()
 
+	table := buildHandlerTable()
 	count := 0
-	for range handlerTable {
+	for range table {
 		count++
 	}