[BUGFIX] Renamed 'response' to 'existing_response' to avoid variable re-declaring

[maxserras]

Description

When checking collisions of responses to avoid duplicate responses given by the same user to the same question, the response argument is redeclared and the response.user_id == response.user_id will always be satisfied, raising an exception.

Closes #5357

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested

Changes done locally, installed the local version and tested on top of our Argilla server with this piece of code (details were obfuscated):

for user in client.users:
       record_to_add.responses.add(rg.Response("label_name", label, user_id=user.id, status="submitted"))

[codecov]

Codecov Report

Attention: Patch coverage is 99.53704% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.21%. Comparing base (8206d55) to head (7385297).
Report is 3 commits behind head on releases/2.0.1.

Files	Patch %	Lines
...lla-server/src/argilla_server/contexts/datasets.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##           releases/2.0.1    #5383      +/-   ##
==================================================
+ Coverage           91.13%   91.21%   +0.08%     
==================================================
  Files                 139      139              
  Lines                5716     5746      +30     
==================================================
+ Hits                 5209     5241      +32     
+ Misses                507      505       -2     

Flag	Coverage Δ
argilla-server	91.21% <99.53%> (+0.08%)	⬆️

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[davidberenstein1957]

@burtenshaw, we will merge this in develop and then cherry-pick from there, right?