test(e2e): stabilize MariaDB 11 tests and fix restore empty-mode client selection

- add `make clean` and run it before `test-e2e` to avoid stale artifacts
- restore: do not hardcode `mysql` for --empty; use detected mariadb/mysql client
- e2e: improve subprocess error output for easier CI debugging
- e2e: adjust MariaDB readiness checks for socket-only root on MariaDB 11
- e2e: add `wait_for_mariadb_sql` and run SQL readiness checks for dedicated TCP user
- e2e: update MariaDB full/no-copy tests to use dedicated user over TCP and consistent credentials

https://chatgpt.com/share/694ecfb8-f8a8-800f-a1c9-a5f410d4ba02

Makefile

@@ -29,6 +29,9 @@ build:
 	@echo ">> Building Docker image $(IMAGE)"
 	docker build -t $(IMAGE) .
 
+clean:
+	git clean -fdX .
+
 # ------------------------------------------------------------
 # Run E2E tests inside the container (Docker socket required)
 # ------------------------------------------------------------
@@ -37,5 +40,5 @@ build:
 # - starts a DinD daemon container on a dedicated network
 # - loads the freshly built image into DinD
 # - runs the unittest suite inside a container that talks to DinD via DOCKER_HOST
-test-e2e: build
+test-e2e: clean build
 	@bash scripts/test-e2e.sh