test(core): add tests for CheckPoint::push and insert methods

test(core): add tests for CheckPoint::push and insert methods

Add comprehensive tests for CheckPoint::push error cases:
- Push fails when height is not greater than current
- Push fails when prev_blockhash conflicts with self
- Push succeeds when prev_blockhash matches

Include tests for CheckPoint::insert conflict handling:
- Insert with conflicting prev_blockhash
- Insert purges conflicting tail
- Insert between conflicting checkpoints

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Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: valued mammal <valuedmammal@protonmail.com>