Validate bootstrap strategy at the API edge, not deep in planning

The previous arrangement only rejected unknown values inside bootstrap planning, which meant typos sat unnoticed on non-bootstrap code paths (Probe, Fetch) and surfaced confusingly late on bootstrap paths after CLI parse and config plumbing had already accepted them. Reject at AdvancedOptions.Validate(), called as the first thing in every Client method that accepts options. Define exported BootstrapStrategyFirstParent / BootstrapStrategyTopo constants so callers don't have to repeat string literals.