fix(validators): always propagate exceptions, never convert to error.execution

Validators operate in the transition-selection phase, not the execution
phase. A validator rejecting is semantically "this transition should not
happen" — not "something failed during execution". Converting the
exception to error.execution conflated these two concepts.

Engine change: pass on_error=None for validators in _conditions_match()
(both sync and async), so exceptions always propagate to the caller
regardless of error_on_execution.

- Add 28 dedicated validator tests (14 scenarios × sync/async via
  sm_runner): propagation, multiple validators, combined with cond,
  no error.execution trigger, no fallthrough
- Rewrite guards.md: progressive narrative, conditions with kwargs
  (PriorityRouter), validator propagation semantics, contrast with
  error.execution, rename title to "Conditions"
- Remove now-unnecessary error_on_execution=False from conftest fixture
  and issue509 test

Author: fgmacedo