[DOP-23867] Implement model -> json converter for tests

[dolfinus]

Change Summary

Most of test lines are JSON output examples, and there are A LOT of them. It is very cumbersome to apply any changes to these tests (like change API schema) because this requires a lot of copy-pasting.

Instead, implement simple functions to convert SQLAlchemy model to JSON representation, and use for all API tests instead of raw JSON examples.

This reduces number of lines a lot (-4k). Also fixes flaky tests caused by datetime isoformat comparison, like 2025-01-01T11:22:33.000000Z != 2025-01-01T11:22:33Z.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.16%. Comparing base (a874b07) to head (124a89c).
Report is 134 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #163   +/-   ##
========================================
  Coverage    93.15%   93.16%           
========================================
  Files          184      185    +1     
  Lines         4208     4228   +20     
  Branches       281      281           
========================================
+ Hits          3920     3939   +19     
- Misses         226      227    +1     
  Partials        62       62           

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