fix: pandas 3 compatibility

[lewisjared]

Description

Resolve pandas FutureWarnings and compatibility issues to support both pandas 2 and 3:

• constraints.py: Skip pd.concat when supplementary_group is empty; drop all-NA columns before concat to avoid dtype determination changes in pandas 3
• example.py: Drop time_bnds before weighted mean (not after) to prevent dtype=object division error from resample+mean on datetime bounds; set explicit compat="override" on open_mfdataset
• test_solve_helpers.py: Normalize None/NaT nulls in parquet round-trip assertions to handle stricter equality checks in pandas 3
• Upgraded lockfile to the latest dependencies

Checklist

Please confirm that this pull request has done the following:

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

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ate-ref-example/src/climate_ref_example/example.py	50.00%	0 Missing and 1 partial ⚠️

Flag	Coverage Δ
core	93.22% <100.00%> (+0.03%)	⬆️
providers	89.56% <87.50%> (-0.09%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...imate-ref-core/src/climate_ref_core/constraints.py	96.74% <100.00%> (+0.03%)	⬆️
...limate-ref-core/src/climate_ref_core/esgf/cmip7.py	93.80% <ø> (ø)
...ool/src/climate_ref_esmvaltool/diagnostics/base.py	95.38% <100.00%> (ø)
...limate-ref-ilamb/src/climate_ref_ilamb/standard.py	90.40% <100.00%> (+0.04%)	⬆️
...ate-ref-example/src/climate_ref_example/example.py	89.79% <50.00%> (-3.83%)	⬇️

... and 3 files with indirect coverage changes

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[lewisjared]

@nocollier This will cause a merge conflict with #548, but we can resolve when merging

[lewisjared]

Pre-req for #506