test(utils): add boundary tests for Older::check_older and After::check_after

[KrishnaShuk]

Description

Addresses the // TODO: test >= / > on line 111 of utils.rs. Adds 11 boundary-condition tests for the Satisfier impls of Older and After.

Notes to the reviewers

Uses Satisfier::<PublicKey>::check_older(...) fully-qualified syntax because the trait is generic over Pk.

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[busayo-OD]

absolute::LockTime can also represent Unix timestamps (>= 500_000_000), but After only has current_height and no current_time. So a timestamp-based locktime would end up being compared against a block height. I also noticed the existing tests don't cover this. Is this intentionally limited to height-based locktimes for now?

[busayo-OD]

I traced the satisfaction flow further. Timestamp CLTV finalization works; existing wallet tests cover that path, and PsbtInputSatisfier::check_after() evaluates the transaction's lock_time via is_implied_by, which is locktime-type aware.

One thing I noticed: After::check_after() does current_height >= n.to_consensus_u32(), which doesn't distinguish between height- and timestamp-based locktimes. Existing timestamp CLTV tests still pass through the signing/finalization flow.