session: fix dml_batch_size doesn't load the global variable (#24710)

[ti-srebot]

cherry-pick #24710 to release-4.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/24730

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---

What problem does this PR solve?

Issue Number: close #24709

Problem Summary:
No TiDBDMLBatchSize in builtinGlobalVariable. So when loading the global variable, it misses TiDBDMLBatchSize.

What is changed and how it works?

Add TiDBDMLBatchSize to builtinGlobalVariable.

How it Works:

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Side effects

Release note

• fix dml_batch_size doesn't load the global variable

[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@wjhuang2016 you're already a collaborator in bot's repo.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• tiancaiamao
• xiongjiwei

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[cfzjywxk]

@wjhuang2016
Seems this fix is missing in the bug triage document?

[tiancaiamao]

/merge

[ti-chi-bot]

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

Commit hash: a875434

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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/run-all-tests

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[wjhuang2016]

/hold

[tiancaiamao]

dml_batch_size is a session variable in 4.0, not a global one, so 4.0 doesn't need this fix.

[wjhuang2016]

Close this issue since dml_batch_size is a session variable.