fix(integration_test): mutex-protect mdb.DB swap in integrationDB helper

The integrationDB helper hot-swaps mdb.DB without mutex protection.
With the new GetDB() RLock, t.Cleanup goroutines writing mdb.DB = prev
race against production goroutines calling GetDB(). Fix: acquire mu.Lock
before the swap and in the Cleanup closure.

workspace-server/internal/handlers/delegation_ledger_integration_test.go

@@ -77,12 +77,18 @@ func integrationDB(t *testing.T) *sql.DB {
 	if _, err := conn.ExecContext(ctx2, `DELETE FROM delegations`); err != nil {
 		t.Fatalf("cleanup: %v", err)
 	}
-	// Wire the package-level db.GetDB() so production helpers (recordLedgerInsert,
-	// recordLedgerStatus) see the same connection.
+	// Wire the package-level db.DB so production helpers (recordLedgerInsert,
+	// recordLedgerStatus) see the same connection. Guard the swap with mu.Lock
+	// to prevent races with production goroutines that call GetDB() (which
+	// acquires RLock) while t.Cleanup runs concurrently.
 	prev := mdb.DB
+	mdb.mu.Lock()
 	mdb.DB = conn
+	mdb.mu.Unlock()
 	t.Cleanup(func() {
+		mdb.mu.Lock()
 		mdb.DB = prev
+		mdb.mu.Unlock()
 		conn.Close()
 	})
 	return conn