This is a minimal PR to check that the message-ix-model test suite runs using ixmp4 storage (i.e. without the ixmp.JDBCBackend and underlying Java code). To do this, it:

• Installs from the branch for Implement ixmp4 shim ixmp#552 that:
  • Provides a minimal 'shim' layer between the behaviour of ixmp.Platform, ixmp.TimeSeries, ixmp.Scenario and the underlying ixmp4 API.
  • Ensures that all of the behaviour of those 3 classes continues to pass the ixmp test suite when using ixmp4 storage. (message-ix-models and downstream/user code rely on that behaviour.)
  • Will (but does not yet, as of 2024-11-28) parametrize the ixmp.testing.test_mp fixture to yield either the current ixmp.JDBCBackend-backed test Platform, or an ixmp4-backed test Platform.
• Installs from the branch for Test against ixmp4 message_ix#894. (Technically this should not be necessary: if the shim works, then the message_ix.tests will pass with the code on message_ix main branch. However, using that branch makes for smaller adjustments to the GitHub Actions workflows here.)
• Confirms that the collective behaviour of the upstream packages continues to pass the message_ix_models.tests suite, with either storage option. (MESSAGEix-GLOBIOM workflows and downstream/user code rely on this behaviour.)

Implementation/to dos:

• As a consequence of parametrizing ixmp.testing.test_mp, all tests in message_ix_models.tests that use the test_mp fixture will run twice: once with JDBCBackend, and once with ixmp4.
• Some other tests may not be parametrized automatically by the upstream changes:
  • Identify these.
  • Parametrize them manually.
• Rebase this branch frequently.
• Debug installation of ixmp4 alongside other dependencies.

Some things not in scope for this PR:

• Changes to the behaviour/API of message_ix_models.
• Migrating code from ixmp or message_ix.

How to review

TBA

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.