Prevent deserialisation error when fetching data from GDA embedded visit service

[DiamondJoseph]

During beamline testing on i22 December 10th, it was found that the current visit service impl uses the wrong method to convert the return value of the GDA numTracker

Instructions to reviewer on how to test:

1. Run a scan that uses GDA as the numTracker service
2. Confirm that the data_session can be deserialised

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[DiamondJoseph]

model_validate_json is for turning a JSON string into the object. model_validate is for turning a Python object. response.json() converts the JSON string returned by the numTracker service into a dict.

[rtuck99]

Please can you fix the tests

[rtuck99]

See above

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.71%. Comparing base (2d409cd) to head (54a372d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #956   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files         166      166           
  Lines        6707     6707           
=======================================
  Hits         6554     6554           
  Misses        153      153           

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[rtuck99]

Approved