ttl: rollback the scan trasnaction even if the BEGIN failed. (#58943)

[ti-chi-bot]

This is an automated cherry-pick of #58943

What problem does this PR solve?

Issue Number: close #58900

Problem Summary:

Previously, the RunInTxn will only rollback the transaction after the BEGIN runs successfully. However, if the BEGIN is killed after setting the transaction status, it can still leave a valid transaction. We'll need to also ROLLBACK it.

The effect is that when we are cancelling a TTL task (scaling the workers, stepping out of the TTL window, stopping the TTL function...), a session with valid transaction may pollute the session pool.

What changed and how does it work?

Move the defer ... ROLLBACK logic to the top of the RunInTxn function.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Release note

Fix the issue that canceling a TTL task may return a polluted session to global session pool.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
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Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	b960100	link	true	/test unit-test
idc-jenkins-ci-tidb/build	b960100	link	true	/test build
idc-jenkins-ci-tidb/check_dev	b960100	link	true	/test check-dev
idc-jenkins-ci-tidb/check_dev_2	b960100	link	true	/test check-dev2

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