Fix MySQL tests - add fallback to environment variables if sharedContainer is nil

If sharedContainer is nil (which shouldn't happen but can in edge cases),
fall back to using environment variables that TestMain sets. This makes
the code more resilient and works consistently for both MySQL and TiDB.

Author: zph

mysql/testcontainers_helper.go

@@ -202,10 +202,20 @@ func getSharedMySQLContainer(t *testing.T, image string) *MySQLTestContainer {
 	}
 
 	// TestMain should have already created sharedContainer
-	// If it's nil, something went wrong in TestMain
+	// If it's nil, fall back to using environment variables (TestMain sets these)
 	if sharedContainer == nil {
-		t.Fatalf("ERROR: sharedContainer is nil. TestMain should have created it using DOCKER_IMAGE='%s'.\n"+
-			"This indicates a problem with TestMain initialization.", dockerImage)
+		endpoint := os.Getenv("MYSQL_ENDPOINT")
+		if endpoint == "" {
+			t.Fatalf("ERROR: sharedContainer is nil and MYSQL_ENDPOINT is not set. TestMain should have created the container or set environment variables using DOCKER_IMAGE='%s'.\n"+
+				"This indicates a problem with TestMain initialization.", dockerImage)
+		}
+		// Fallback: use environment variables set by TestMain
+		return &MySQLTestContainer{
+			Container: nil, // Not available, but tests use environment variables
+			Endpoint:  endpoint,
+			Username:  os.Getenv("MYSQL_USERNAME"),
+			Password:  os.Getenv("MYSQL_PASSWORD"),
+		}
 	}
 
 	return sharedContainer