session: fix auto commit variable change effect on transaction commit (#36631)

[ti-srebot]

cherry-pick #36631 to release-4.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/36748

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git push git@github.com:ti-srebot/tidb.git pr/36748:release-4.0-1cd8c31b1ace

---

What problem does this PR solve?

Issue Number: close #36581

Problem Summary:

What is changed and how it works?

When changing the value of the session variable auto-commit, the implicit commit for ongoing transactions should happen only when it's changed from off to on.

Check List

Tests

• Unit test

Side effects

Documentation

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix the auto-commit mode change related transaction commit behaviours.

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-srebot]

/run-all-tests

[ti-srebot]

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[xhebox]

close for DMR version