variable: disable tidb_read_staleness for SET_VAR hint

[winoros]

What problem does this PR solve?

Issue Number: close #48478

Problem Summary:

What is changed and how it works?

This variable takes effect at the planner's preprocess phase. But we build SET_VAR hint just before we parse the AST node.

We disable it first to make the behavior consistent.

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

Merging #48477 (3706c18) into master (3956421) will increase coverage by 1.5575%.
Report is 31 commits behind head on master.
The diff coverage is n/a.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #48477        +/-   ##
================================================
+ Coverage   71.3861%   72.9436%   +1.5575%     
================================================
  Files          1402       1427        +25     
  Lines        406824     414716      +7892     
================================================
+ Hits         290416     302509     +12093     
+ Misses        96474      93278      -3196     
+ Partials      19934      18929      -1005     

Flag	Coverage Δ
integration	43.3479% <ø> (?)
unit	71.5607% <ø> (+0.1745%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9874% <ø> (ø)
parser	∅ <ø> (∅)
br	48.8773% <ø> (-4.0736%)	⬇️

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [AilinKid,hawkingrei]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-09 09:24:06.113684993 +0000 UTC m=+3722643.700795138: ☑️ agreed by AilinKid.
• 2023-11-09 10:28:11.722992233 +0000 UTC m=+3726489.310102373: ☑️ agreed by hawkingrei.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #48491.