Fix incorrect meta->data_returned for paginated results with MongoDB

[ml-evs]

Closes #1140.

[codecov]

Codecov Report

Merging #1141 (5b62eda) into master (5db4452) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1141   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files          68       68           
  Lines        3888     3889    +1     
=======================================
+ Hits         3592     3593    +1     
  Misses        296      296           

Flag	Coverage Δ
project	92.38% <100.00%> (+<0.01%)	⬆️
validator	92.38% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
optimade/server/entry_collections/mongo.py	94.54% <100.00%> (+0.10%)	⬆️

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[ml-evs]

Had to add a few tricks to get the immutable_id stuff working, so actually going to split this into 2 PRs.

[ml-evs]

The remaining changes now are pretty trivial (and already reviewed once by @JPBergsma) so I will self-merge when tests pass.

[ml-evs]

Annoyingly all the tests are failing due to an under-pinned dependency in aiida-core (namely, upf-to-json). I hate that we have to rely on AiiDA and verdi setup (due to its complexity) etc. working in our CI...

[ml-evs]

AiiDA pins to upf-to-json~=0.9.2 and tests upf-to-json==0.9.3 in its CI. upf-to-json==0.9.4 just came out and seems to break things, so I have pinned back to 0.9.2. We are already pinned to an older AiiDA version (1.6, not 2) so doubt this will be backported.