tracker(dm): fix downstream table structure reports key too long (#7014)

[ti-chi-bot]

This is an automated cherry-pick of #7014

Signed-off-by: lance6716 lance6716@gmail.com

What problem does this PR solve?

Issue Number: close #5315

What is changed and how it works?

during the refactor of schema tracker, I found this issue is not hard to fix

Check List

Tests

• Unit test

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

Fix a problem that DM will report `Specified key was too long` error

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[lance6716]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: c1335cd

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