Add support for HGVS insertions of form {pos}_{pos}ins{pos}_{pos}

> NM_004006.2:c.849_850ins858_895
the insertion of a copy of nucleotids c.858 to c.895 between nuclotides c.849 and c.850

from https://varnomen.hgvs.org/recommendations/DNA/variant/insertion/

Our tokenizer provided support for this, but our validator and translator does not support this query. Our service returns an internal server error.

Start by tracing the existing tokenizer support for HGVS insertions through the validator and translator, then reproduce the example query from the issue against the service. Update the validation and translation path so c.849_850ins858_895 is handled without an internal server error, and verify the service returns the expected normalized result.

Written by the indexing model from the issue text.