ttl: fix the timezone issue and panic in the caller of getSession (#58166)

[ti-chi-bot]

This is an automated cherry-pick of #58166

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

[YangKeao]

/retest

[codecov]

Codecov Report

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

Please upload report for BASE (release-8.5@4468f56). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #58709   +/-   ##
================================================
  Coverage               ?   57.0199%           
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  Files                  ?       1770           
  Lines                  ?     627244           
  Branches               ?          0           
================================================
  Hits                   ?     357654           
  Misses                 ?     245423           
  Partials               ?      24167           

Flag	Coverage Δ
integration	36.8777% <50.0000%> (?)
unit	72.7047% <50.0000%> (?)

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

Components	Coverage Δ
dumpling	52.8711% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	52.2712% <0.0000%> (?)

[YangKeao]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-01-06 09:28:32.151565969 +0000 UTC m=+173055.440397674: ☑️ agreed by lcwangchao.
• 2025-01-07 06:02:59.939229063 +0000 UTC m=+247123.228060768: ☑️ agreed by YangKeao.

[lcwangchao]

/merge

[ti-chi-bot]

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[lcwangchao]

/approve

[ti-chi-bot]

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