The in-flight-messages guard prevented silent misbehavior, but the
companion Workflow.reset() escape hatch only cleared _messages while
leaving iteration count, executor-local state, and shared State
mutations in an indeterminate condition after a mid-run failure. That
gave a false sense of recovery.
Recovery from a mid-run failure is supported only via checkpoint
restoration. Keep the guard and reframe its error message accordingly;
remove reset() and its tests.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>