Harden subprocessor filters per review feedback

Address the review comments on the subprocessors work:

- Reject invalid category/country filter values in the Subprocessors
  resolver with an INVALID error instead of relying solely on transport
  coercion, so a malformed request fails fast and explicitly.
- Use pgx.StrictNamedArgs in the new distinct facet queries so missing
  or extra SQL placeholders stay detectable, matching sibling queries.
- Default a nil ThirdPartyFilter at the service boundary to avoid a nil
  dereference in the coredata list/count paths.
- Expose the category group label as an aria heading for assistive tech.
- Add the missing space in the Select "Selected:" story label.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/trust/third_party_service.go

@@ -60,6 +60,10 @@ func (s ThirdPartyService) ListForOrganizationId(
 	cursor *page.Cursor[coredata.ThirdPartyOrderField],
 	filter *coredata.ThirdPartyFilter,
 ) (*page.Page[*coredata.ThirdParty, coredata.ThirdPartyOrderField], error) {
+	if filter == nil {
+		filter = coredata.NewThirdPartyFilter(nil, nil, nil, nil, nil)
+	}
+
 	var thirdParties coredata.ThirdParties
 
 	err := s.svc.pg.WithConn(