variable: always truncate tidb_session_alias when it is not a valid value

[lcwangchao]

What problem does this PR solve?

Issue Number: ref #46071

What is changed and how it works?

after discussing, we need always truncate value when setting tidb_session_alias even if the value is not valid.

Check List

Tests

• Unit test
• 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

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[codecov]

Codecov Report

Merging #46707 (50315c0) into master (6e7f36b) will increase coverage by 0.8638%.
Report is 85 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #46707        +/-   ##
================================================
+ Coverage   73.3586%   74.2225%   +0.8638%     
================================================
  Files          1315       1350        +35     
  Lines        395675     415634     +19959     
================================================
+ Hits         290262     308494     +18232     
- Misses        86932      88712      +1780     
+ Partials      18481      18428        -53     

Flag	Coverage Δ
integration	28.7789% <57.1428%> (?)
unit	74.8765% <100.0000%> (+1.5178%)	⬆️

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

Components	Coverage Δ
dumpling	54.0444% <ø> (ø)
parser	87.0740% <ø> (+2.1158%)	⬆️
br	50.0241% <ø> (-2.4134%)	⬇️

[lcwangchao]

/retest

[lcwangchao]

/retest

[lcwangchao]

/retest

[bb7133]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-07 09:30:18.136530442 +0000 UTC m=+2610582.685546414: ☑️ agreed by CbcWestwolf.
• 2023-09-11 09:00:26.725724252 +0000 UTC m=+259808.650280632: ☑️ agreed by bb7133.

[easonn7]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bb7133, CbcWestwolf, easonn7

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Needs approval from an approver in each of these files:

• OWNERS [CbcWestwolf,bb7133]
• sessionctx/variable/OWNERS [easonn7]

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[lcwangchao]

/retest