Decode-drop recovery stress test uses undefined harness credentials

The decode-drop recovery stress test passes `ctx.nodeA.HDB_ADMIN_USERNAME` and `ctx.nodeA.HDB_ADMIN_PASSWORD` to its initial `add_node` call. Those are not integration-harness node properties, so the operation serializes empty credentials and fails before the test reaches the decode-drop assertions.

Replace the undefined properties with the harness-provided `ctx.nodeA.admin` object at each relevant cluster join. The stress job must then prove its intended behavior: clean records flow past poisoned records, poisoned records are absent, the decode-close loop does not occur, and a post-join live write replicates.