A crashed worker used to poison the rest of the run. execute() reported the
crash as a failed result, but kept the dead session, and every later call then
raised RuntimeError out of the tool. Clear the session so the next call checks
out a replacement. Keep the session for a syntax or runtime error, which leaves
the worker healthy, and say in the failure text that a restart loses the
variables.
A MemoryFile accepts writes, so metadata.json took them while the other three
document files refused. Mount it through the same read and deny pair. The
write-denial test now covers all four files.
94befc0dfd