session, variable: cleanup TiDBEvolvePlanBaselines sysvar validation

[morgo]

What problem does this PR solve?

Problem Summary:

This is a cleanup following #26333

What is changed and how it works?

What's Changed:

In the PR #26333 validation for the sysvar was performed in the functions SetGlobalSysVar and SetSessionFromHook. But the API for sysvars allows for a validation function to be specified in the sysvar struct. Handling it this way is preferred because then plugins have access to exactly the same API. It also keeps the sysvar handling centralized and consistent.

Check List

Tests

• Unit test (covered by existing tests)
• 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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• xhebox
• xiongjiwei

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[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: 15c6541

[Reminiscent]

Good catch! Thanks for your contribution. @morgo