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]

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@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/build	d59aa10	link	true	/test build
idc-jenkins-ci-tidb/check_dev	d59aa10	link	true	/test check-dev
idc-jenkins-ci-tidb/unit-test	d59aa10	link	true	/test unit-test

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Please upload report for BASE (release-8.5@920eb53). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #58951   +/-   ##
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  Coverage               ?   15.5458%           
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  Files                  ?       1629           
  Lines                  ?     598313           
  Branches               ?          0           
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  Hits                   ?      93013           
  Misses                 ?     492313           
  Partials               ?      12987           

Flag	Coverage Δ
integration	15.5458% <0.0000%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	∅ <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	2.7456% <0.0000%> (?)