Add test to make sure appends create conversation field

[lferran]

Description

[sc-14709]

How was this PR tested?

Describe how you tested this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.21%. Comparing base (ee0ca16) to head (a2912d6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3650      +/-   ##
==========================================
+ Coverage   85.18%   85.21%   +0.02%     
==========================================
  Files         563      563              
  Lines       48465    48465              
  Branches    14173    14173              
==========================================
+ Hits        41287    41300      +13     
+ Misses       6560     6548      -12     
+ Partials      618      617       -1     

Flag	Coverage Δ
nidx	79.75% <ø> (+<0.01%)	⬆️
nucliadb	73.55% <100.00%> (+0.01%)	⬆️
nucliadb-ingest	43.60% <0.00%> (ø)
nucliadb-reader	43.64% <50.00%> (+0.03%)	⬆️
nucliadb-search	54.69% <0.00%> (ø)
nucliadb-standalone	46.01% <0.00%> (ø)
nucliadb-train	44.67% <0.00%> (ø)
nucliadb-writer	46.94% <0.00%> (+<0.01%)	⬆️
nucliadb_dataset	73.65% <ø> (ø)
nucliadb_models	71.55% <ø> (ø)
nucliadb_sdk	83.08% <ø> (ø)
nucliadb_telemetry	72.33% <ø> (ø)
nucliadb_utils	80.94% <ø> (ø)

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