Fix Windows GRIB splitting without grib_copy

[CyrilJl]

Summary

• Fix Windows handling of GRIB files larger than 2 GiB without relying on the external grib_copy executable.
• Split large GRIB files through the Python ecCodes API before opening per-variable datasets with cfgrib.

Main changes

• Add a shortName-based splitter in meteofetch/_model.py using codes_grib_new_from_file, codes_get, codes_write, and codes_release.
• Keep cfgrib import lazy so unit tests can mock the reader without requiring the native ecCodes library at import time.
• Add focused tests for message grouping and the Windows large-file read path.

Validation

• rtk uv run --extra test pytest -q tests/test_model_grib_split.py
• rtk python3 -m compileall meteofetch tests
• rtk proxy git diff --check