test: sample current-process RSS before query execution

[VVKot]

Which issue does this PR close?

Local reproduction of a zero-RSS memory-validation failure. Depends on the window-expression lint prerequisite: #26207.

Rationale for this change

A short query can finish before the asynchronous RSS monitor starts, leaving max_rss at zero and failing the memory-limit validation. System::new_all also gathers information unrelated to the current process.

What changes are included in this PR?

Capture current-process memory before running the query, initialize the maximum from that sample, and reuse the same System in the monitor. Both refreshes exclude task enumeration. Memory thresholds are unchanged.

What is the testing strategy for this PR?

The previously failing sort_with_mem_limit_1_runner now passes. The existing memory_limit_validation_runner_works_runner still rejects its impossible 20 MB cap. The core integration suite passed 1,227 tests with one ignored. The extended workspace run is in progress.

Are there any user-facing changes?

None; this changes test infrastructure.

AI-assisted contribution authored with Codex.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.80%. Comparing base (75ddead) to head (f15043c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26208      +/-   ##
==========================================
- Coverage   82.80%   82.80%   -0.01%     
==========================================
  Files        1149     1149              
  Lines      452205   452205              
  Branches   452205   452205              
==========================================
- Hits       374458   374448      -10     
- Misses      54950    54958       +8     
- Partials    22797    22799       +2     

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