feat(chain)!: make `CheckPoint` `data` field optional#2024

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LagginTimes wants to merge 6 commits intobitcoindevkit:masterfrom

LagginTimes:optional_data

Commits on Sep 28, 2025

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

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LagginTimes
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Commits on Oct 3, 2025

fix(core): fix `CheckPoint::insert` and `CheckPoint::push` logic
LagginTimes
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