Add GeneMapper integration to ClinGenStreamer

[enriquea]

Summary

Integrate the GeneMapper utility with ClinGenStreamer to enable automatic gene ID translation when querying ClinGen data.

Background

The HGNC Gene ID Mapping feature was implemented, which provides the GeneMapper utility for bidirectional gene ID mapping. The ClinGen integration was deferred to a follow-up.

Proposed Changes

Extend ClinGenStreamer to accept an optional GeneMapper instance for ID translation:

clingen = ClinGenStreamer(clingen_ht)
mapper = GeneMapper(hgnc_ht)

# Query by Ensembl ID (translated via mapper)
genes = clingen.get_genes_by_classification(
    "Definitive",
    gene_mapper=mapper,
    output_id_type="ensembl_gene_id"
)

Implementation Tasks

• Add optional gene_mapper parameter to relevant ClinGenStreamer methods
• Add output_id_type parameter to control output format (hgnc_id, gene_symbol, ensembl_gene_id, etc.)
• Add input ID translation for methods that accept gene identifiers
• Add tests for mapper integration
• Update documentation with usage examples

Reference

See implementation plan: docs/planning/HGNC-Mapper-Implementation.md (Integration Points section)