ddl: fix a bug about setting MaxIndexLength

[zimulala]

What problem does this PR solve?

Issue Number: close https://github.com/pingcap/tidb/issues/53281

Problem Summary: Both scenarios are explained in the issue.

Introduce code for this issue: https://github.com/pingcap/tidb/pull/50624/files#diff-45ad039a8b87835b475a52615355c444538d3a1f31a84d7b3ac94e15b1ce8721R126-R127

What changed and how does it work?

Check List

Tests

• [x] 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

[tiprow[bot]]

Hi @zimulala. Thanks for your PR.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.9529%. Comparing base (6aef624) to head (8b1dce0). Report is 7 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #53284 +/- ## ================================================ + Coverage 72.4435% 74.9529% +2.5093% ================================================ Files 1493 1497 +4 Lines 429362 432661 +3299 ================================================ + Hits 311045 324292 +13247 + Misses 99037 88435 -10602 - Partials 19280 19934 +654 ``` | [Flag](https://app.codecov.io/gh/pingcap/tidb/pull/53284/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/pingcap/tidb/pull/53284/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `49.8580% <100.0000%> (?)` | | | [unit](https://app.codecov.io/gh/pingcap/tidb/pull/53284/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `71.7219% <100.0000%> (+0.3875%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap#carryforward-flags-in-the-pull-request-comment) to find out more. | [Components](https://app.codecov.io/gh/pingcap/tidb/pull/53284/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | Coverage Δ | | |---|---|---| | [dumpling](https://app.codecov.io/gh/pingcap/tidb/pull/53284/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `53.9957% <ø> (ø)` | | | [parser](https://app.codecov.io/gh/pingcap/tidb/pull/53284/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `∅ <ø> (∅)` | | | [br](https://app.codecov.io/gh/pingcap/tidb/pull/53284/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `50.4818% <ø> (+9.0483%)` | :arrow_up: |

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[pkg/ddl/OWNERS](https://github.com/pingcap/tidb/blob/master/pkg/ddl/OWNERS)~~ [tangenta] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-05-15 04:26:21.364238863 +0000 UTC m=+1627335.121374435: :ballot_box_with_check: agreed by Defined2014.
• 2024-05-15 06:08:18.337950202 +0000 UTC m=+1633452.095085776: :ballot_box_with_check: agreed by tangenta.

[zimulala]

/retest-required

[tiprow[bot]]

@zimulala: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to [this](https://github.com/pingcap/tidb/pull/53284#issuecomment-2111953482): >/retest-required Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository.

[bb7133]

/run-cherry-picker

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #53408.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #53409.