variable: unifiy MaxExecuteTime usage and fix some problem (#50915)

[ti-chi-bot]

This is an automated cherry-pick of #50915

What problem does this PR solve?

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linking the relevant issues via the "close" or "ref".

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Issue Number: close #50914, close #50872

Problem Summary:

What changed and how does it work?

1. provide a Function GetMaxExecutionTime for SessionVars.
2. Fix two potential bug, caused by not properly used MaxExecutionTime, one about plan_stats，the other expression/build_in.
3. Fix a bug related with tidb_stats_load_sync_wait
4. unify code for GetMaxExecutionTime.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• 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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, qw4990

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• OWNERS [AilinKid,qw4990]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-11-11 12:41:17.503359797 +0000 UTC m=+273639.694228779: ☑️ agreed by qw4990.
• 2024-11-11 13:13:43.750949315 +0000 UTC m=+275585.941818323: ☑️ agreed by AilinKid.