Fix SHOW QUERIES and KILL QUERY privilege filtering

[shuwenwei]

Summary

• Restrict tree-model SHOW QUERIES results to the current user when the user does not satisfy the MAINTAIN authorization check.
• Restrict tree-model KILL QUERY to the current user's queries under the same condition.
• Allow users with the SYSTEM privilege, which contains the deprecated MAINTAIN privilege, to view and kill queries from all users.
• Add IoTDBShowQueriesIT and IoTDBKillQueryIT with queries that remain active across Session fetches.

Root cause

TreeAccessCheckVisitor.visitShowQueries and visitKillQuery set allowedUsername when the MAINTAIN authorization check succeeded instead of when it failed. As a result, an unprivileged user could see or kill other users' queries, while a privileged user could be unnecessarily restricted.

Impact

Tree-model SHOW QUERIES and KILL QUERY now apply the intended visibility and control rules while retaining the existing MAINTAIN authorization check.

Validation

• Added integration tests that leave result sets unconsumed and verify query visibility and kill permissions before and after granting SYSTEM; the tests do not grant deprecated MAINTAIN directly.
• Tests were not run locally, following the repository instruction not to proactively run build or test checks unless requested.

[sonarqubecloud]

Quality Gate failed

Failed conditions
C Reliability Rating on New Code (required ≥ A)

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.39%. Comparing base (a8497be) to head (b1ce540).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...an/relational/security/TreeAccessCheckVisitor.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18359      +/-   ##
============================================
- Coverage     43.40%   43.39%   -0.01%     
  Complexity      374      374              
============================================
  Files          5366     5366              
  Lines        382885   382885              
  Branches      49796    49796              
============================================
- Hits         166187   166171      -16     
- Misses       216698   216714      +16     

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