v0.2.0: get_sequence length field + exactly-one input contracts

get_sequence length field + exactly-one input contracts

Two breaking changes to the tool surface; consumers must update on upgrade.

Breaking

• ensembl_get_sequence output field lengthInBp renamed to length — every caller received this field.
• ensembl_get_homology and ensembl_lookup_gene now reject a call providing more than one of symbol/id/ids/symbols; previously one input silently won.

Changed

• ensembl_get_sequence format() labels length by molecule type (residues for protein, bp for genomic/cdna/cds) instead of bp/aa. (#4)
• Conflicting inputs now fail with a conflicting_input ValidationError on both tools. (#5)
• symbol/species descriptions corrected: species defaults to homo_sapiens for symbol lookups rather than being required. (#5)

94 tests pass; bun run devcheck clean.