executor: support showing session_states when signing cert is unavailable

[djshow832]

What problem does this PR solve?

Issue Number: close #59336

Problem Summary:
Some users only want TiProxy to replay traffic, which needs session states but not session tokens.
However, show session_states reports errors when the signing cert is unavailable. Installing signing certs requires TiDB to restart, but users don't want that.

What changed and how does it work?

Return NULL in the token column if the signing cert is unavailable.

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

[tiprow]

Hi @djshow832. Thanks for your PR.

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[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 73.4674%. Comparing base (a69f863) to head (4eddad7).
Report is 48 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #59337        +/-   ##
================================================
+ Coverage   73.1091%   73.4674%   +0.3582%     
================================================
  Files          1689       1690         +1     
  Lines        466728     472948      +6220     
================================================
+ Hits         341221     347463      +6242     
+ Misses       104552     104318       -234     
- Partials      20955      21167       +212     

Flag	Coverage Δ
integration	42.9417% <0.0000%> (?)
unit	72.2550% <60.0000%> (-0.0442%)	⬇️

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

Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	44.6202% <ø> (-0.6829%)	⬇️

[bb7133]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bb7133, xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-08 23:54:10.284601952 +0000 UTC m=+141492.680824009: ☑️ agreed by xhebox.
• 2025-02-09 02:33:07.973655089 +0000 UTC m=+151030.369877151: ☑️ agreed by bb7133.