[IMP] account_reconciliation_widget: reduce heavy queries count

This specific query is quite expensive, and what's worse, it's also done twice: once to count records and once to get them.

By using a window function, we can get the count and results in one query, thus almost doubling performance in this operation.

Related resources:

- [Explanation](https://stackoverflow.com/a/22353886/1468388).
- [Explain of the 1st SELECT COUNT(*), before the patch](https://explain.dalibo.com/plan/TgL).
- [Explain of the 2nd SELECT, before the patch](https://explain.dalibo.com/plan/Yc9).
- [Explain of the new, unified SELECT, after the patch](https://explain.dalibo.com/plan/9MA).

@Tecnativa TT28595 TT29916 https://github.com/odoo/odoo/pull/73765

account_reconciliation_widget/models/reconciliation_widget.py

@@ -102,14 +102,13 @@ class AccountReconciliation(models.AbstractModel):
             search_str=search_str,
             mode=mode,
         )
-        recs_count = self.env["account.move.line"].search_count(domain)
 
         from_clause, where_clause, where_clause_params = (
             self.env["account.move.line"]._where_calc(domain).get_sql()
         )
         query_str = sql.SQL(
             """
-            SELECT "account_move_line".id FROM {from_clause}
+            SELECT "account_move_line".id, COUNT(*) OVER() FROM {from_clause}
             {where_str}
             ORDER BY ("account_move_line".debit -
                       "account_move_line".credit) = {amount} DESC,
@@ -129,7 +128,11 @@ class AccountReconciliation(models.AbstractModel):
         self.env["account.bank.statement"].flush()
         self._cr.execute(query_str, params)
         res = self._cr.fetchall()
-
+        try:
+            # All records will have the same count value, just get the 1st one
+            recs_count = res[0][1]
+        except IndexError:
+            recs_count = 0
         aml_recs = self.env["account.move.line"].browse([i[0] for i in res])
         target_currency = (
             st_line.currency_id