Apply coredata rules: idempotent deletes and constraint checks

Delete methods no longer check RowsAffected — deletes are
idempotent. PgError handlers now check both error code and
constraint name to avoid misattributing violations. Also
migrated remaining errors.As patterns to errors.AsType.

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

pkg/coredata/applicability_statement.go

@@ -250,9 +250,8 @@ VALUES (
 
 	_, err := conn.Exec(ctx, q, args)
 	if err != nil {
-		var pgErr *pgconn.PgError
-		if errors.As(err, &pgErr) {
-			if pgErr.Code == "23505" {
+		if pgErr, ok := errors.AsType[*pgconn.PgError](err); ok {
+			if pgErr.Code == "23505" && pgErr.ConstraintName == "states_of_applicability_contr_state_of_applicability_id_con_key" {
 				return ErrResourceAlreadyExists
 			}
 		}
@@ -382,15 +381,11 @@ WHERE
 	args := pgx.StrictNamedArgs{"id": applicabilityStatementID}
 	maps.Copy(args, scope.SQLArguments())
 
-	result, err := conn.Exec(ctx, q, args)
+	_, err := conn.Exec(ctx, q, args)
 	if err != nil {
 		return fmt.Errorf("cannot delete applicability statement: %w", err)
 	}
 
-	if result.RowsAffected() == 0 {
-		return ErrResourceNotFound
-	}
-
 	return nil
 }