Fix knowledge compiler Go port regressions (MySQL 1101 + Start deadlock) (#17599)

Two regressions from #17536: (1) MySQL 8.0 AutoMigrate crash on
knowledge_compile_docs (Error 1101) from a TEXT default; (2) ingestor
Start() sync.Once re-entrant deadlock. Both fixed with regression tests.

internal/engine/global.go

@@ -88,6 +88,13 @@ func GetMessageQueueEngine() MessageQueue {
 	return messageQueueEngine
 }
 
+// SetMessageQueueEngine installs the global message-queue engine. It exists
+// primarily as a test seam so callers can drive Start() without a real server
+// config; production code uses InitMessageQueueEngine.
+func SetMessageQueueEngine(mq MessageQueue) {
+	messageQueueEngine = mq
+}
+
 func InitMessageQueueEngine(messageQueueType string) error {
 	config := server.GetConfig()
 	switch messageQueueType {