fix: apply version filtering and fix supplementary dataset selection in regression analysis

[lewisjared]

Description

Fixes two issues in the dataset selection logic that caused spurious entries in regression analysis results:

1. Version filtering in dataset queries (datasets/base.py): Added proper version filtering to query_datasets and query_facets methods. Previously, version constraints were silently ignored, causing older dataset versions to leak into results. The version filter supports both exact matching and ordering comparisons (e.g. v20190731 vs v20200101).

2. Supplementary dataset selection (constraints.py): Fixed AddSupplementaryDataset to pick a single best-matching dataset per group instead of accumulating multiple candidates across score ties. Previously, when multiple supplementary datasets tied on matching score, all were selected, leading to spurious experiment entries (e.g. 1pctCO2, esm-1pct-brch-1000PgC) appearing alongside expected historical and SSP data.

These fixes together eliminate unexpected experiment/version combinations from regression outputs.

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[lewisjared]

@bouweandela what do you think about 1cb40c0 ?

[lewisjared]

Closes #545 and #543

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
core	92.51% <100.00%> (+0.04%)	⬆️
providers	89.65% <ø> (ø)

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Files with missing lines	Coverage Δ
...imate-ref-core/src/climate_ref_core/constraints.py	96.75% <100.00%> (+0.04%)	⬆️
...kages/climate-ref/src/climate_ref/datasets/base.py	98.47% <100.00%> (+0.04%)	⬆️
...kages/climate-ref/src/climate_ref/solve_helpers.py	98.92% <100.00%> (+0.02%)	⬆️

... and 1 file with indirect coverage changes

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