lightning: pick the first file to check schema (#27607)

[ti-srebot]

cherry-pick #27607 to release-5.2
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/27623

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---

What problem does this PR solve?

Issue Number: close ##27605
Problem Summary:
check all files costs too much time.

What is changed and how it works?

What's Changed:
pick one random file to check schema is valid.

Check List

Tests

• Unit test

Release note

Fix the issue that pre-check cost too much time when import too many files for tables.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• kennytm

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[ti-srebot]

/run-all-tests

[ti-srebot]

@3pointer you're already a collaborator in bot's repo.

[glorv]

@zhouqiang-cl PTAL

[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: 5e56243

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