fix: FIT-991: OOM on CSV async_export

[matt-bernstein]

followup to #5612 fixing last site where this occurs

**NOTE: this is not necessarily the root cause of the related TRIAG bug; that still has to be established. It's something I noticed along the way.

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[codecov]

Codecov Report

❌ Patch coverage is 58.82353% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.66%. Comparing base (0a7db05) to head (fe5bcc7).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
label_studio/data_export/api.py	46.15%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #8820       +/-   ##
============================================
+ Coverage    67.71%   80.66%   +12.95%     
============================================
  Files          806      253      -553     
  Lines        62093    23114    -38979     
  Branches     10328        0    -10328     
============================================
- Hits         42044    18646    -23398     
+ Misses       20046     4468    -15578     
+ Partials         3        0        -3     

Flag	Coverage Δ
lsf-e2e	?
lsf-integration	?
lsf-unit	?
pytests	80.66% <58.82%> (-0.01%)	⬇️

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[matt-bernstein]

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