[IMP] mail: change relation of partner_id field

This commit helps reducing the noise of the model definition PR. Indeed,
the partner_id field on the mail.follower model was mocked as an integer
field. The /mail/thread/data route returns an integer. But the search
read method returns an array for many2one fields [id, display_name]. Thus,
using the real server field will result in an incorrect behavior.

task-2767820

closes odoo/odoo#86096

Related: odoo/enterprise#25125
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/src/components/chatter_topbar/tests/chatter_topbar_tests.js

@@ -403,10 +403,11 @@ QUnit.test('composer state conserved when clicking on another topbar button', as
 QUnit.test('rendering with multiple partner followers', async function (assert) {
     assert.expect(7);
 
-    this.data['res.partner'].records.push({
-        id: 100,
-        message_follower_ids: [1, 2],
-    });
+    this.data['res.partner'].records.push(
+        { id: 11 },
+        { id: 12 },
+        { id: 100, message_follower_ids: [1, 2] },
+    );
     this.data['mail.followers'].records.push(
         {
             // simulate real return from RPC

addons/mail/static/src/components/follower_list_menu/tests/follower_list_menu_tests.js

@@ -148,7 +148,7 @@ QUnit.test('click on "add followers" button', async function (assert) {
         partner.message_follower_ids.push(1);
         payload.options.on_close();
     });
-    this.data['res.partner'].records.push({ id: 100 });
+    this.data['res.partner'].records.push({ id: 100 }, { id: 42 });
     this.data['mail.followers'].records.push({
         partner_id: 42,
         email: "bla@bla.bla",

addons/mail/static/tests/helpers/mock_models.js

@@ -117,7 +117,7 @@ export class MockModels {
                     is_active: { type: 'boolean' },
                     is_editable: { type: 'boolean' },
                     name: { type: 'char' },
-                    partner_id: { type: 'integer' },
+                    partner_id: { relation: 'res.partner', type: 'many2one' },
                     res_id: { type: 'many2one_reference' },
                     res_model: { type: 'char' },
                     subtype_ids: { type: 'many2many', relation: 'mail.message.subtype' }

addons/mail/static/tests/helpers/mock_server.js

@@ -541,6 +541,9 @@ MockServer.include({
                         ['res_id', '=', thread.id],
                         ['res_model', '=', thread_model],
             ]], {});
+            // search read returns many2one relations as an array [id, display_name].
+            // But the original route does not. Thus, we need to change it now.
+            followers.forEach(follower => follower.partner_id = follower.partner_id[0]);
             res['followers'] = followers;
         }
         if (request_list.includes('suggestedRecipients')) {