fix(core): correct bound_into_included_u64 handling for excluded start bounds

[yappermoar-boop]

Updated bound_into_included_u64 in core/src/utils/mod.rs to correctly perform saturating_add(1) when handling an Excluded start bound (is_start == true). Previously, saturating_sub(1) was executed regardless of is_start, producing incorrect lower bounds. Added unit tests to verify behavior for all Bound variants.

[github-actions]

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[huitseeker]

At u64::MAX, the new addition saturates back to u64::MAX, so an excluded start becomes a range containing that value instead of an empty range. Excluding 0 as an end has the same problem. Could this return Option or Result to represent these empty bounds and cover both endpoints here?