[FIX] sustainability_purchase: Fix traceback with partner merging wizard

The native partner merging wizard doesn't support fields of type `reference` because it is never used by Odoo on model `res.partner`.
Because we add a `reference` field on `res.partner`, we got a traceback using this wizard on a record with this field filled. This commit fix it.

C.f. ONS(project.task,115597)

sustainability_purchase/__init__.py

@@ -1,4 +1,5 @@
 from . import models
+from . import wizards
 
 
 def add_carbon_mode_columns(env):

sustainability_purchase/wizards/__init__.py

@@ -0,0 +1 @@
+from . import base_partner_merge

sustainability_purchase/wizards/base_partner_merge.py

@@ -0,0 +1,75 @@
+import itertools
+import logging
+from collections import defaultdict
+
+from odoo import api, models
+from odoo.exceptions import ValidationError
+
+_logger = logging.getLogger("odoo.addons.base.partner.merge")
+
+
+class BasePartnerMergeAutomaticWizard(models.TransientModel):
+    _inherit = "base.partner.merge.automatic.wizard"
+
+    @api.model
+    def _update_values(self, src_partners, dst_partner):
+        """Copied from
+        https://github.com/odoo/odoo/blob/9d4f8b2fda5c75438ad30a08b4f7d95eac4f2f23/odoo/addons/base/wizard/base_partner_merge.py#L240
+
+        The native function can't merge fields of type `reference` because it is never user by Odoo on model `res.partner`.
+        In this addon, we use a `reference` field on `res.parnter`, so this wizard needs to support them.
+        """
+        _logger.debug(
+            "_update_values for dst_partner: %s for src_partners: %r",
+            dst_partner.id,
+            src_partners.ids,
+        )
+
+        model_fields = dst_partner.fields_get().keys()
+        summable_fields = self._get_summable_fields()
+
+        def write_serializer(item):
+            if isinstance(item, models.BaseModel):
+                return item.id
+            else:
+                return item
+
+        # get all fields that are not computed or x2many
+        values = dict()
+        values_by_company = defaultdict(dict)  # {company: vals}
+        for column in model_fields:
+            field = dst_partner._fields[column]
+            if field.type not in ("many2many", "one2many") and field.compute is None:
+                for item in itertools.chain(src_partners, [dst_partner]):
+                    if item[column]:
+                        if field.type == "reference":
+                            values[column] = f"{item[column]._name},{item[column].id}"
+                        elif column in summable_fields and values.get(column):
+                            values[column] += write_serializer(item[column])
+                        else:
+                            values[column] = write_serializer(item[column])
+            elif field.company_dependent and column in summable_fields:
+                # sum the values of partners for each company; use sudo() to
+                # compute the sum on all companies, including forbidden ones
+                partners = (src_partners + dst_partner).sudo()
+                for company in self.env["res.company"].sudo().search([]):
+                    values_by_company[company][column] = sum(
+                        partners.with_company(company).mapped(column)
+                    )
+
+        # remove fields that can not be updated (id and parent_id)
+        values.pop("id", None)
+        parent_id = values.pop("parent_id", None)
+        dst_partner.write(values)
+        for company, vals in values_by_company.items():
+            dst_partner.with_company(company).sudo().write(vals)
+        # try to update the parent_id
+        if parent_id and parent_id != dst_partner.id:
+            try:
+                dst_partner.write({"parent_id": parent_id})
+            except ValidationError:
+                _logger.info(
+                    "Skip recursive partner hierarchies for parent_id %s of partner: %s",
+                    parent_id,
+                    dst_partner.id,
+                )