fix(variables): Variable.where works with solution attached

[FabianHofmann]

Note

This PR's description and code changes were generated with AI assistance (Claude Code) and reviewed by a human.

Changes proposed in this Pull Request

Variable.where raised ValueError: exact match required for all data variable names as soon as a solution was attached to the model (after Model.solve) or the variable was fixed.

Cause: Variable.where passes a fill dict covering only labels/lower/upper to Dataset.where, which requires the fill keys to exactly match the dataset's data variables. After solving (or fix()), the variable's data gains extra columns (solution, _stashed_lower, _stashed_upper), so the match fails.

Fix: Augment the fill dict to cover every data variable present, defaulting extras to NaN. The explicit labels=-1 / lower=upper=NaN entries still override; only auxiliary columns pick up the NaN default.

This routes through Variable.fillna too, since it delegates to where. Audited the related methods — ffill/bfill (Dataset.where(cond) + Dataset.fillna(dict)) and shift (Dataset.shift(fill_value=dict)) already tolerate extra data variables, and LinearExpression/QuadraticExpression/Constraint are unaffected (they don't store solution/dual).

Added a regression test test_variable_where_with_solution and a release note.

Checklist

• AI-generated content is marked (see AGENTS.md).
• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[codspeed-hq]

Merging this PR will not alter performance

✅ 138 untouched benchmarks
⏩ 138 skipped benchmarks¹

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_{Comparing fix/variable-where-with-solution (e3c50c7) with master (d810de7)}

Footnotes

1. 138 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩