ddl: avoid sysvar drift in admin check before drop

[fzzf678]

What problem does this PR solve?

Issue Number: close #65969

Problem Summary:

adminCheckTableBeforeDrop may call ExecRestrictedSQL multiple times. Each call can fetch a different internal session from the system session pool, so the sequence SET tidb_enable_fast_table_check=ON / ADMIN CHECK TABLE / restore may land on different sessions. This can break ADMIN CHECK TABLE on features that require fast check (e.g. partial indexes) and may leave the session pool in a polluted state.

The previous code also restored tidb_enable_fast_table_check using the user session's value, which can unintentionally propagate a user setting into the internal session pool.

What changed and how does it work?

• Run the whole SET ON -> ADMIN CHECK TABLE -> restore sequence on a single internal session acquired once from DDL's session pool.
• Restore tidb_enable_fast_table_check to the original value from the same internal session (instead of reading it from the user session).
• Add (*session.Pool).Destroy() to discard internal sessions that should not be reused (e.g. sysvar change fails), while keeping the pool slot consistent for both DestroyableSessionPool and *pools.ResourcePool.
• Add regression test that sets up mixed tidb_enable_fast_table_check values in the sys session pool and verifies DROP TABLE succeeds with partial indexes when check-table-before-drop is enabled.
• Add unit tests covering Destroy() semantics for both pool types.

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]

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[codecov]

Codecov Report

❌ Patch coverage is 77.35849% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.8924%. Comparing base (1e2cdaa) to head (a6a4430).
⚠️ Report is 145 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #65972        +/-   ##
================================================
+ Coverage   77.8543%   78.8924%   +1.0380%     
================================================
  Files          1961       1954         -7     
  Lines        539718     554664     +14946     
================================================
+ Hits         420194     437588     +17394     
+ Misses       117868     115588      -2280     
+ Partials       1656       1488       -168     

Flag	Coverage Δ
integration	47.4199% <33.9622%> (-0.7818%)	⬇️
unit	76.7045% <77.3584%> (+0.2303%)	⬆️

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Components	Coverage Δ
dumpling	56.7974% <ø> (+0.1783%)	⬆️
parser	∅ <ø> (∅)
br	61.8904% <ø> (+0.7730%)	⬆️

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[ti-chi-bot]

@fzzf678: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-unit-test-ddlv1	a6a4430	link	true	/test pull-unit-test-ddlv1

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