planner: add missing distinct flag for Apply convert to join (#26959)

[ti-srebot]

cherry-pick #26959 to release-5.1
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/26969

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tidb.git pr/26969:release-5.1-7afab6eafb26

---

What problem does this PR solve?

Issue Number: close #26958

Problem Summary:

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

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 add a release note, or a 'None' if it is not needed.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Reminiscent
• qw4990

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[ti-srebot]

/run-all-tests

[ti-srebot]

@wshwsh12 you're already a collaborator in bot's repo.

[purelind]

/run-all-tests

[qw4990]

/merge

[ti-chi-bot]

@qw4990: /merge in this pull request requires 2 approval(s).

In response to this:

/merge

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[Reminiscent]

/merge

[ti-chi-bot]

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

Commit hash: b4a9bb2