Fallback to any_state when state-specific guard fails

[vasdal]

Problem:

• When a state-specific transition exists but its guard condition returns false, the state machine does not fall back to try any_state (_) transitions for the same event
• The event is consumed without any action being executed, even when a matching any_state transition exists

Solution:

• Modified get_state_mapping in include/boost/sml.hpp to try any_state transitions when state-specific transitions fail
• Uses aux::conditional_t to select at compile-time: combined_mapping (with || fallback) when any_state has real mappings, otherwise state_mapping directly
• This avoids double-triggering unexpected_event handlers
• Added test case any_state_fallback_when_guard_fails in test/ft/states.cpp to verify the fix

Reviewers:
@kris-jusiak