ci(optional): stop uv run from reinstalling the removed dependencies

[jdhughes-dev]

uv run re-syncs the project before running the command, which undoes both the
uv sync --only-group test install and the random uv pip uninstall that precede it. Both
matrix variants therefore run the full suite with every optional dependency present, so the
workflow has not been testing the absence of any of them.

Evidence from the 22 Aug run (32562777097):
the no optional dependencies job logs optional packages: affine-3.0.0, ..., h5py-3.16.0, ..., pymetis-2025.2.2, ... and passes test_model_splitter.py::test_multi_model, which requires pymetis.

• run pytest with --no-sync so the environment the preceding steps built is the one used
• add a workflow_dispatch trigger so the workflow can be run on demand

Opened as a draft: once the constraint is actually enforced this may surface tests that do not
degrade gracefully, which is the point of the workflow, but I cannot exercise it from a branch —
workflow_dispatch only appears once the trigger is on the default branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.3%. Comparing base (556c088) to head (e2ccae5).
⚠️ Report is 209 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2823      +/-   ##
===========================================
+ Coverage     55.5%    73.3%   +17.8%     
===========================================
  Files          644      659      +15     
  Lines       124135   132188    +8053     
===========================================
+ Hits         68947    97013   +28066     
+ Misses       55188    35175   -20013     

see 585 files with indirect coverage changes

🚀 New features to boost your workflow:

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