test(core): clean up test structure

[niklashigi]

Note

Standardizes test mocks to use status_code, simplifies request body capture and respx lifecycle, and adds a POST body serialization test.

• Test utilities (tests/conftest.py):
  • Standardize mocked response key to status_code and set default headers for JSON bodies.
  • Simplify request body capture to parse JSON from request.content for non-GET requests.
  • Manage respx via an autouse reset_respx fixture; MockContext.clear() now only clears routes and captured requests.
• Tests:
  • Migrate all mocks to status_code and remove redundant isinstance assertions in error tests.
  • Add test_should_correctly_serialize_post_request_body validating POST body serialization.
  • Minor wording fix in test_job_board docstring.

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