Classification export: accept the 'a' allele shortcut in allele= param (e.g. a9678) and validate

[davmlaw]

🤖 Written by Claude

The classification export API accepts a debug allele= query param which is parsed with a bare int(allele_str) in ClassificationFilter.from_request (classification/views/exports/classification_export_filter.py:476). A non-numeric value raises ValueError and 500s the export endpoint.

Crucially, a is the allele shortcut used in search (e.g. searching a9678 resolves allele 9678), so users naturally pass allele=a9678. Rather than just rejecting it, the export form should accept that shortcut:

• strip a leading a (so a9678 and 9678 both resolve to allele 9678), consistent with search;
• validate the remainder is an integer;
• return a clean 400 for genuinely invalid input instead of a 500.

Seen on shariant / sharianttest:

• https://app.rollbar.com/a/jimmy.andrews/fix/item/VariantGrid/7040
• https://app.rollbar.com/a/jimmy.andrews/fix/item/VariantGrid/7041

Split out of #1601 (which now covers only the genome-build and grouping-id 404 cases).

[davmlaw]

Shariant testing required

• Enter an allele ID as an integer, with "a" prefix (ie "a101") and as a bad value