Add unified.cpu-threshold config

[v01dstar]

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What is the related PR or file link(s)?

• This PR is translated from: Add unified.cpu-threshold config docs#22167
• Other reference link(s): unified read pool: cpu-threshold on unified read-pool tikv/tikv#18948

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

[ti-chi-bot]

Hi @v01dstar. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[qiancai]

/ok-to-test

[qiancai]

Rest LGTM

[Oreoxmt]

/cc @Oreoxmt

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-12-17 08:36:56.329375837 +0000 UTC m=+1634961.143153399: ☑️ agreed by qiancai.
• 2026-01-13 02:59:58.682779781 +0000 UTC m=+326442.744644691: ☑️ agreed by Oreoxmt.

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #21267.