ttl: fix the timezone issue and panic in the caller of getSession

[YangKeao]

What problem does this PR solve?

Issue Number: close #58107, close #58109

It fixes two issues:

1. The caller of getSession() may panic, because the getSession() may return nil, nil.
2. The timezone of the now in rescheduleJobs is incorrect.

What changed and how does it work?

1. Remove the nowInGlobalTZ in rescheduleJobs. It's introduced in https://github.com/pingcap/tidb/pull/51863/files#diff-9053b8d1ea0965da6e623343fd8fa0706cf9fc01a327e70a00aa7d7ae376177eR568, but I don't know why. It seems that the timeutil.WithinDayTimePeriod will set location to UTC instantly.
2. Replace a err = to err := in getSession()

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

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Defined2014, lcwangchao

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-12-11 08:39:29.52504098 +0000 UTC m=+427759.613843522: ☑️ agreed by lcwangchao.
• 2024-12-11 08:50:48.992193214 +0000 UTC m=+428439.080995758: ☑️ agreed by Defined2014.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.5913%. Comparing base (0ffac36) to head (60f398a).
Report is 194 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #58166        +/-   ##
================================================
+ Coverage   73.1496%   73.5913%   +0.4417%     
================================================
  Files          1675       1675                
  Lines        461848     463644      +1796     
================================================
+ Hits         337840     341202      +3362     
+ Misses       103258     101654      -1604     
- Partials      20750      20788        +38     

Flag	Coverage Δ
integration	43.1806% <50.0000%> (?)
unit	72.3721% <50.0000%> (+0.0912%)	⬆️

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Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	45.4877% <ø> (-0.4986%)	⬇️

[hawkingrei]

/retest

[tiprow]

@YangKeao: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
fast_test_tiprow	60f398a	link	true	/test fast_test_tiprow

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #58709.