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/coredata/third_party.go

@@ -800,7 +800,7 @@ ORDER BY
 `
 	q = fmt.Sprintf(q, scope.SQLFragment())
 
-	args := pgx.NamedArgs{"organization_id": organizationID}
+	args := pgx.StrictNamedArgs{"organization_id": organizationID}
 	maps.Copy(args, scope.SQLArguments())
 
 	rows, err := conn.Query(ctx, q, args)
@@ -836,7 +836,7 @@ ORDER BY
 `
 	q = fmt.Sprintf(q, scope.SQLFragment())
 
-	args := pgx.NamedArgs{"organization_id": organizationID}
+	args := pgx.StrictNamedArgs{"organization_id": organizationID}
 	maps.Copy(args, scope.SQLArguments())
 
 	rows, err := conn.Query(ctx, q, args)