Adjust for pandas-stubs 2.3.3.251201

[khaeru]

Pandas-stubs v2.3.3.251201 was released (per the version) on 2025-12-01. This led to mypy errors in the "Code quality" job of the "pytest" CI workflow.

The PR adjusts:

• Scenario.add_par() is adjusted in line with Adjust for pandas-stubs 2.3.3.251201 ixmp#615.
• 3 new mypy errors in add_year/__init__.py are ignored. These lines of code are not covered by tests, so it is difficult to confirm what the runtime (rather than implied) types of variables are and make adjustments. The lines are likely to be changed or removed as part of other PRs targeting this module (Install 3rd party type stubs for mypy #484, Fix Scenario errors in add_year() #986).

How to review

Read the diff and 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, type hinting only
• Update release notes. ditto

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.2%. Comparing base (26aebd4) to head (0ce4246).
⚠️ Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix/tools/add_year/__init__.py	0.0%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #992   +/-   ##
=====================================
  Coverage   92.2%   92.2%           
=====================================
  Files         60      60           
  Lines       5155    5155           
=====================================
  Hits        4756    4756           
  Misses       399     399           

Files with missing lines	Coverage Δ
message_ix/core.py	93.4% <ø> (ø)
message_ix/tools/add_year/__init__.py	66.7% <0.0%> (ø)

[khaeru]

The CI checks are visible on this run triggered by the "labeled" event, rather than the "pull_request_target" event. All pass.

We accept the codecov/patch check failure because, as noted:

These lines of code [in add_year/__init__.py] are not covered by tests