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

[ti-srebot]

cherry-pick #36631 to release-5.2
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/36751

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/36751:release-5.2-1cd8c31b1ace

---

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]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[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.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[ti-srebot]

/run-all-tests

[VelocityLight]

This pull request is closed because it's related version has closed automatic cherry-picking.
If it's still needed, you can reopen it or just regenerate it using bot,
see:

• https://prow.tidb.io/command-help#cherrypick
• https://book.prow.tidb.net/#/plugins/cherrypicker

You can find more details at:

• https://internals.tidb.io/t/topic/785