[FIX] mrp: MTO - decrease the post production when decreasing the SO

Use case to reproduce:
- Create a product with manufacture and MTO route
- Set the warehouse in 3 steps manufacturing
- Confirm a SO for 5 units
- Update the quantity to 4 units

Current behavior
Each picking has a quantity to 4 units. Except for the post
production to stock that still for 5 units but a return is
created.

Expected behavior
Each picking has 4 units

It happens due to `_run_pull` that will create a new procurement
group in post production location (the purpose is to avoid a merge
between semi-finished product and finished product). But in the case
of a negative procurement, we want to decrease a picking and not
create a production order. So create a new procurement group with the
next MO sequence will aslo create another issue because
`_run_manufacture` will skip the MO creation since the quantity is
negative and it will be a gap in the sequence number.

Instead of creating a new procurement group, search for an existing
in sibbling moves, so `_search_picking_for_assignation` could find
a picking to merge the negative procurement and decrease an existing
move.

closes odoo/odoo#100439

X-original-commit: c5e8adaa2b364e3ff4625e080df4c523a9673668
Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>

addons/mrp/models/stock_rule.py

@@ -83,6 +83,10 @@ class StockRule(models.Model):
             if not warehouse_id:
                 warehouse_id = rule.location_dest_id.warehouse_id
             if rule.picking_type_id == warehouse_id.sam_type_id:
+                if float_compare(procurement.product_qty, 0, precision_rounding=procurement.product_uom.rounding) < 0:
+                    procurement.values['group_id'] = procurement.values['group_id'].stock_move_ids.filtered(
+                        lambda m: m.state not in ['done', 'cancel']).move_orig_ids.group_id[:1]
+                    continue
                 manu_type_id = warehouse_id.manu_type_id
                 if manu_type_id:
                     name = manu_type_id.sequence_id.next_by_id()