One-shot latch for an irrecoverably invalid payload.
Serves two roles. It keeps the warning to one line per payload, even though
parse_args re-runs on every later chunk for the retained buffer. It also
short-circuits parse_args itself: every condition that sets it is
permanent (see _warn_invalid_args), so re-running json.loads per
fragment could only fail again — which is what made the pre-incremental
version quadratic on a malformed payload.
That second role means the latch now gates data, not just logging: keep it
private to _warn_invalid_args, since setting it for any other reason would
silently drop a valid tool call. Its lifetime must match the payload whose
parse it blocks, so _reset_args_fragment_state clears it alongside the
fragment state.
