Cache message-static-data in "pytest" workflow

[khaeru]

This adjusts the workflow in order to reduce Git LFS bandwidth usage.

In particular:

1. The message-static-data repo is checked out only once for each OS in the warm-lfs-cache job. This uses some of Git LFS bandwidth quota.
2. The resulting files are stored in the GitHub Actions cache.
3. The files are restored in the matrixed pytest job (5 per OS) to the same paths.

Neither of steps (2) or (3) count towards Git LFS bandwidth, so this should reduce our run rate.

Also:

• Adjust for Implement ixmp4 shim ixmp#552: These changes were obviated by Adjust dependency on ixmp4 ixmp#570.
  • Temporarily disable the dependency on ixmp[tests]. This is because ixmp[tests] → ixmp[ixmp4] → ixmp4 via a Git URL. This indirect dependency on a URL is not supported, and so install fails.
  • Add .types.PlatformArgs and use in .util.config.Config.
• Xfail two tests that fail with these changes. These will be addressed in Transport improvements from 2025-W15 #328.

How to review

• Read the diff.
• Note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A; CI changes
• Update doc/whatsnew.
• (Once approved) Drop the TEMPORARY commit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.7%. Comparing base (3986245) to head (eab5347).
⚠️ Report is 767 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #339     +/-   ##
=======================================
- Coverage   78.6%   77.7%   -0.9%     
=======================================
  Files        215     215             
  Lines      17219   17220      +1     
=======================================
- Hits       13536   13389    -147     
- Misses      3683    3831    +148     

Files with missing lines	Coverage Δ
message_ix_models/model/transport/testing.py	100.0% <ø> (ø)
...age_ix_models/tests/model/transport/test_report.py	92.3% <100.0%> (+<0.1%)	⬆️
message_ix_models/types.py	100.0% <ø> (ø)
message_ix_models/util/config.py	98.7% <100.0%> (ø)

... and 7 files with indirect coverage changes

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[khaeru]

I decided some of these things need to flow back to ixmp itself so we aren't making the same adjustments in multiple places → iiasa/ixmp#570. Will drop the commits from this branch once that is in.

[khaeru]

Will drop the commits from this branch once that is in.

Okay, now done, and here is a workflow run that passes. I am not sure why some of the run times appear longer today.

I'll now rebase to drop the temporary commit so the PR is ready to merge.