v2: merge .core and .problem

[dweindl]

Move v2.problem.* to v2.core. Keep all pydantic models in the same module. Avoid unnecessary import complexity.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.30151% with 158 lines in your changes missing coverage. Please review.

Project coverage is 73.83%. Comparing base (11db39d) to head (726b247).

Files with missing lines	Patch %	Lines
petab/v2/core.py	60.20%	122 Missing and 36 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #414      +/-   ##
==========================================
- Coverage   73.88%   73.83%   -0.06%     
==========================================
  Files          62       61       -1     
  Lines        6771     6756      -15     
  Branches     1196     1196              
==========================================
- Hits         5003     4988      -15     
  Misses       1308     1308              
  Partials      460      460              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.