planner: Add subquery variables

[Oreoxmt]

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What is changed, added or deleted? (Required)

Which TiDB version(s) do your changes apply to? (Required)

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• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
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What is the related PR or file link(s)?

pingcap/tidb#63204
pingcap/tidb#63416

• This PR is translated from: planner: Add subquery variables docs#21943
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[Oreoxmt]

/cc @qiancai

[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-11-04 08:09:47.60205743 +0000 UTC m=+171237.045087299: ☑️ agreed by qiancai.

[Oreoxmt]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Oreoxmt

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #21056.