test: add unit tests for Variant, ReferenceMapping, Haplotype, and _parse_pop_freqs

[ameynert]

Summary

Extends tests/tools/test_remap_divref.py with direct unit tests for methods and a private helper that were previously only exercised indirectly through Haplotype.reference_mapping():

• Variant.render() — SNP, insertion, deletion
• Haplotype.parsed_variants() — single variant, multiple variants, caching (second call returns the same list object with no re-parsing)
• Haplotype.contig() — returns chromosome of first variant
• ReferenceMapping.variants_involved_str() — empty list, single variant, multiple variants
• _parse_pop_freqs() — all floats, mixed nulls converted to 0.0, single null

None of these require Hail; all are pure Python.

Test plan

• uv run --directory divref poe check-all passes
• All 13 new tests pass

🤖 Generated with Claude Code

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✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch am_15_test_remap_divref_models

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