Update add_AFOLU_CO2_accounting() per message_data:ssp_dev

[khaeru]

Per this thread (Slack, private), the version of add_AFOLU_CO2_accounting() on the message_data dev branch, migrated here in #350, no longer works with current scenarios like ixmp://ixmp-dev/SSP_SSP1_v5.0/baseline_DEFAULT_step_13. This manifests as breakage in the .model.transport workflow, with exceptions like:

ComputationError: computing 'SSP5 policy with tax' using:

<Task 'SSP5 policy with tax' <Step tax_emission()>(...)>

Use Computer.describe(...) to trace the computation.

Computation traceback:
  File "/home/runner/actions-runner/_work/message_data/message_data/.venv/lib/python3.13/site-packages/message_ix_models/workflow.py", line 125, in __call__
    result = self.action(context, s, **self.kwargs)
  File "/home/runner/actions-runner/_work/message_data/message_data/.venv/lib/python3.13/site-packages/message_ix_models/model/transport/workflow.py", line 161, in tax_emission
    scenario = engage_workflow.step_0(context, scenario)
  File "/home/runner/actions-runner/_work/message_data/message_data/.venv/lib/python3.13/site-packages/message_data/projects/engage/workflow.py", line 188, in step_0
    add_AFOLU_CO2_accounting(scenario, **kw)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
  File "/home/runner/actions-runner/_work/message_data/message_data/.venv/lib/python3.13/site-packages/message_data/tools/utilities/add_AFOLU_CO2_accounting.py", line 65, in add_AFOLU_CO2_accounting
    raise ValueError("'land_output' not available for commodity 'LU_CO2'")
ValueError: 'land_output' not available for commodity 'LU_CO2'

This PR updates the version here to match the version on message_data ssp_dev branch, thus partly addresses #275.

Some immediate observations about changes to the code:

• The previous version of the function modified and parameters relation_activity, input. The new version of the code does not.
• The function previously constructed data to be added by reference to the scenario structure. Now the added data is derived from existing scenario contents.
• The function previously passed the constraint_value parameter to add_CO2_emission_constraint(). Now this parameter is totally unused.
• The glb parameter is renamed to glb_reg, but is now ignored.
• The test added in Manual adjustments for .engage.workflow migration #351 fails.
• Thus the docstring of the function, which has not been updated and references "relations" in the summary and body, appears to be out of date and no longer an accurate description.

The PR also tries to address these changes.

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.0%. Comparing base (4bc1cfa) to head (e2a3817).
Report is 3 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #354   +/-   ##
=====================================
  Coverage   72.0%   72.0%           
=====================================
  Files        243     243           
  Lines      19451   19457    +6     
=====================================
+ Hits       14011   14021   +10     
+ Misses      5440    5436    -4     

Files with missing lines	Coverage Δ
message_ix_models/tests/test_tools.py	78.1% <100.0%> (+4.4%)	⬆️
...essage_ix_models/tools/add_AFOLU_CO2_accounting.py	95.6% <100.0%> (+7.8%)	⬆️

... and 3 files with indirect coverage changes

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