ci: add python 3.14 to integration test matrix and update default to 3.14

[mwyau]

This PR adds Python 3.14 to the integration test matrix in .github/workflows/ci.yml and updates the default Python version to 3.14 in .python-version, Dockerfile, and the docs-build CI job. Test coverage collection and upload are now restricted to Python 3.14 for both unit and integration tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.53%. Comparing base (8d95bd8) to head (143bfea).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #99   +/-   ##
=======================================
  Coverage   66.53%   66.53%           
=======================================
  Files          22       22           
  Lines        2059     2059           
=======================================
  Hits         1370     1370           
  Misses        689      689           

Flag	Coverage Δ
integration	60.90% <ø> (ø)
unit	62.06% <ø> (ø)

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