fix failing test test_pipeline_with_documents

[ArneBinder]

#505 did not work out as expected, so we need to try again.

Note that this test fails only on the main branch CI, since it is (was) marked as slow. unfortunately, I can not reproduce locally.

TODO:

• temporarily make test_pipeline_with_documents not slow to run it in PR CI: does not fail here either
• investigate further
• make test_pipeline_with_documents slow again

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.68%. Comparing base (a6bb91d) to head (be540b9).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #506      +/-   ##
==========================================
+ Coverage   76.98%   82.68%   +5.69%     
==========================================
  Files          31       64      +33     
  Lines        1803     5358    +3555     
  Branches      350     1129     +779     
==========================================
+ Hits         1388     4430    +3042     
- Misses        337      675     +338     
- Partials       78      253     +175     

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