feat: add time_units and calendar metadata to CMIP6/CMIP7 datasets

[lewisjared]

Description

Adds time_units and calendar metadata columns to CMIP6 and CMIP7 dataset models, enabling proper handling of non-standard CF calendars (e.g., 360_day, noleap, proleptic_gregorian).

Previously, start_time/end_time were stored as datetime.datetime, which cannot represent dates like Feb 30 in a 360-day calendar. This PR:

• Adds time_units and calendar columns to both CMIP6Dataset and CMIP7Dataset SQLAlchemy models
• Changes DatasetFile.start_time/end_time from DateTime to String column type in the DB
• Extracts calendar metadata from netCDF files via the complete parsers (cmip6_parsers.py, cmip7_parsers.py)
• Replaces parse_datetime with parse_cftime_dates in utils.py, producing cftime.datetime objects that support all CF calendars
• Updates constraint comparisons (PartialDateTime, RequireContiguousTimerange, RequireOverlappingTimerange) to work with cftime.datetime via duck typing and mixed-calendar fallbacks
• Converts times on DB load in the base adapter so all downstream code works with cftime objects
• Handles parquet round-trips by converting cftime to/from strings in solve_helpers.py
• Includes an Alembic migration that adds the new columns and resets finalised=False to trigger re-finalisation

Key design decisions

• cftime.datetime objects stored as strings in DB (cftime doesn't subclass datetime.datetime)
• During partial DRS finalization, mixed calendars ("standard" vs "proleptic_gregorian") are handled via string-based fallback comparison
• climate-ref-core does NOT depend on cftime; PartialDateTime uses duck typing (getattr) instead of isinstance

Checklist

Please confirm that this pull request has done the following:

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

[codecov]

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