fix(ecstore): guard transition worker max (#3003)

* fix(ecstore): guard transition worker max

* test: clarify env mutation safety

* test(ecstore): document env mutation concurrency contract

rustfs/src/app/lifecycle_transition_api_test.rs

@@ -327,6 +327,9 @@ async fn wait_for_transition(
     }
 }
 
+// SAFETY: this helper is used only by `#[serial]` tests and runs under the single-threaded Tokio
+// runtime (`worker_threads = 1`), so no concurrent test can mutate process environment during the
+// `env::set_var` / `env::remove_var` window.
 #[allow(unsafe_code)]
 async fn with_forced_immediate_enqueue_timeout<F, Fut>(test_fn: F)
 where