test(types): add unit tests for PruningConfig enabled and effective_min_height

[lau90eth]

Summary

Add unit tests for PruningConfig to improve test coverage.

The PruningConfig struct had 0% test coverage. These tests verify:

• enabled() returns false by default
• enabled() returns true when certificates_distance > 0
• enabled() returns true when certificates_before > Height::new(0)
• effective_certificates_min_height() prefers certificates_before over certificates_distance
• effective_certificates_min_height() correctly subtracts distance from current height
• effective_certificates_min_height() saturates at zero when distance exceeds current height
• effective_certificates_min_height() returns zero when pruning is disabled

Testing

cargo test -p arc-consensus-types

All 31 tests pass.