mdl: move the set variable check of nextgen from variable pkg to planner

[D3Hunter]

What problem does this PR solve?

Issue Number: ref #61702

Problem Summary:

What changed and how does it work?

in #62865, we forbid setting enableMDL var, but I didn't notice that Domain.rebuildSysVarCache will call SetGlobal directly, which will cause logging of the error setting tidb_enable_metadata_lock is not supported in the next generation of TiDB, although there is no harm, it's not expected, so we move the check to planner of set statement

Check List

Tests

• Unit test, existing test TestSetMDLInNextGen also work
• Integration test
• Manual test (add detailed scripts or steps below)

run nextgen TiDB, there is no log like setting tidb_enable_metadata_lock is not supported in the next generation of TiDB

mysql> set global tidb_enable_metadata_lock=1;
ERROR 1235 (42000): setting tidb_enable_metadata_lock is not supported in the next generation of TiDB

• 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

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[codecov]

Codecov Report

❌ Patch coverage is 2.77778% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.4145%. Comparing base (8acfef8) to head (f7c75cb).
⚠️ Report is 16 commits behind head on master.

⚠️ Current head f7c75cb differs from pull request most recent head 3193383

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Additional details and impacted files

@@               Coverage Diff                @@
##             master     #62882        +/-   ##
================================================
+ Coverage   72.8011%   73.4145%   +0.6134%     
================================================
  Files          1792       1822        +30     
  Lines        487624     495787      +8163     
================================================
+ Hits         354996     363980      +8984     
+ Misses       111008     109467      -1541     
- Partials      21620      22340       +720     

Flag	Coverage Δ
integration	45.0158% <2.7777%> (?)

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Components	Coverage Δ
dumpling	52.8700% <ø> (ø)
parser	∅ <ø> (∅)
br	46.3318% <ø> (+0.0076%)	⬆️

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[D3Hunter]

/retest

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[D3Hunter]

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[D3Hunter]

/retest

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[D3Hunter]

/retest

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[D3Hunter]

/hold

some case keeps failing, investigating, but it runs fine locally, i also see it in other PR, might be some other cases forget to revert the changes to global variables. see #62889

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-08-08 06:20:48.062255343 +0000 UTC m=+594758.205016528: ☑️ agreed by tangenta.
• 2025-08-08 08:42:31.301500054 +0000 UTC m=+603261.444261219: ☑️ agreed by GMHDBJD.

[D3Hunter]

/unhold

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/retest

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/retest

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: GMHDBJD, hawkingrei, qw4990, tangenta, yudongusa

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Details

Needs approval from an approver in each of these files:

• OWNERS [GMHDBJD,hawkingrei,qw4990,tangenta]
• pkg/planner/OWNERS [hawkingrei,qw4990]
• pkg/sessionctx/variable/OWNERS [yudongusa]

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[hawkingrei]

/ok-to-test