test: add tests for rewrite_fasta

[ameynert]

Summary

Adds tests/tools/test_rewrite_fasta.py covering rewrite_fasta() using pytest's tmp_path fixture. No Hail or external data required — all tests use in-memory FASTA strings written to temp files.

Cases covered:

• Canonical autosomes chr1–chr22 are kept
• Sex chromosomes chrX, chrY, and chrMT are kept
• Alt contigs (chr1_alt), unplaced contigs (chrUn_gl000220), and virus sequences (chrEBV) are filtered out
• Mixed input with canonical and non-canonical contigs interleaved
• Empty input produces empty output
• Multi-line sequences under a canonical header are written in full

Test plan

• uv run --directory divref poe check-all passes
• All 6 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_16_test_rewrite_fasta

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