ddl: use another way to get index uniqueness during task exec init (#50378)

[ti-chi-bot]

This is an automated cherry-pick of #50378

What problem does this PR solve?

Issue Number: ref #50307, close #50435

What changed and how does it work?

This PR fixes the stuck issue cause by indexInfo not found when cancelling the task.

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.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[Benjamin2037]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-17 07:19:07.344679207 +0000 UTC m=+340388.908976897: ☑️ agreed by Benjamin2037.
• 2024-01-17 07:19:45.105783646 +0000 UTC m=+340426.670081352: ☑️ agreed by ywqzzy.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Benjamin2037, D3Hunter, ywqzzy

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Needs approval from an approver in each of these files:

• br/pkg/lightning/OWNERS [Benjamin2037,D3Hunter]
• pkg/ddl/OWNERS [Benjamin2037,ywqzzy]

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[tangenta]

/retest

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.6@f573a65). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

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Flag	Coverage Δ
unit	70.0248% <0.0000%> (?)

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br	49.6612% <0.0000%> (?)