Add guards for empty domains in variable bound propagators

[Copilot]

Variable bound propagators accessed range_l[-1] and range_l[0] without checking for empty domains, causing IndexError during constraint solving failures.

Changes

• Fixed variable_bound_bounds_max_propagator.py: Guard range_l[-1] access, return -2^63 on empty domain to force constraint failure
• Fixed variable_bound_bounds_min_propagator.py: Guard range_l[0] access, return 2^63 on empty domain to force constraint failure
• Added test_constraint_rangelist_extended.py: 10 tests covering tuple syntax edge cases (empty domains, boundaries, mixed syntax, overlapping constraints)
• Added test_variable_bound_propagators.py: 7 tests covering propagator behavior with empty domains (chained, offset, circular constraints)

Context

The propagators are called during constraint propagation via:

def propagate(self):
    max_v = self.max()  # Calls subclass max() which accessed range_l[-1] unsafely
    # ... rest of propagation logic

When constraint solving creates impossible conditions (e.g., a > 200 and a in rangelist((0,50))), domains can become empty before propagators run. The guards ensure SolveFailure is properly raised instead of crashing with IndexError.

Original prompt

Investigate, create tests, and confirm reported behavior. Correct issues, using context provided in the issue.

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