Tree: close temporary tar files used by copy and rcopy

[thiell]

TreeWorker never closes its temporary tar files: one per forward copy worker, one per rcopy source node. Their file descriptors stay open until the file objects are garbage-collected, which is deferred by reference cycles — and for a timed-out rcopy node cannot happen before the worker object itself is released. This means ResourceWarning: unclosed file noise in warning-enabled runs and open-fd accumulation in long-running processes embedding the library. Found while writing the tree copy tests for #680.

• copy: wrap the temporary tar file in a with block in _launch()
• rcopy: close the per-node temporary tar file in _on_remote_node_close() — TarFile.close() does not close a caller-provided fileobj
• rcopy: close leftover tar files of timed-out nodes at worker completion in _check_fini(), and drop data arriving after a node's terminal event (the gateway's final grooming flush follows its TimeoutMessage), so the cleanup is final
• add regression tests for these paths (fail before the fix, pass after)