membuffer: improve ART's code and test coverage

[you06]

ref pingcap/tidb#55287 This PR tries to make the code simplify and increase the test coverage (only some unreachable paths are not covered).

[cfzjywxk]

@ekexium PTAL again

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, ekexium

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tikv/client-go/blob/master/OWNERS)~~ [cfzjywxk,ekexium] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-10-29 04:31:11.664201369 +0000 UTC m=+324184.503356909: :ballot_box_with_check: agreed by ekexium.
• 2024-10-30 09:03:18.419791509 +0000 UTC m=+426911.258947320: :ballot_box_with_check: agreed by cfzjywxk.
• 2024-10-30 10:00:00.637676742 +0000 UTC m=+430313.476832288: :heavy_multiplication_x::repeat: reset by you06-pingcap.
• 2024-10-30 12:42:05.032249396 +0000 UTC m=+440037.871404942: :ballot_box_with_check: agreed by cfzjywxk.
• 2024-10-31 01:33:18.219568732 +0000 UTC m=+486311.058724278: :ballot_box_with_check: agreed by ekexium.