refactor(sources): migrate grib source off LegacySource

[frazane]

Description

Migrates the grib source (GribSource) from LegacySource onto the modern Source base class. First step towards #553.

What problem does this change solve?

LegacySource is an intermediate base, kept for historical reasons, that stores *args/**kwargs and dispatches through a @staticmethod _execute, leaving sources split across two base classes. This moves grib onto the standard Source contract — config in __init__, retrieval in execute_valid_dates — mirroring the sibling GribIndexSource, which already does this with the same flavour/grid_definition config. Pure refactor: the _execute body moves verbatim, and the base-class execute_intervals -> execute_valid_dates fallback is unchanged.

What issue or task does this change relate to?

Related to #553.

Additional notes

Behaviour-preserving: building the same recipe with the pre- and post-change grib.py produces a bit-identical dataset (data, statistics, coordinates, dates, variables). test_grib passes unchanged.

This change was developed in tandem with AI coding agents.

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[aaron-hopkinson]

Looks fine to me - one optional/minor comment.

Thanks for doing this.