[FIX] calendar: event from activity strip text from html

From a SO (for example), create an activity with type meeting
Create the calendar event

Access it as a guest with the route with the token

Before this commit, there was an error 500 because the description
of the event actually contained "\n" and was not parseable

After this commit, we strip the string at event creation, and no error occur

OPW 1867283
closes #25795

addons/calendar/models/mail_activity.py

@@ -24,7 +24,7 @@ class MailActivity(models.Model):
             'default_res_id': self.env.context.get('default_res_id'),
             'default_res_model': self.env.context.get('default_res_model'),
             'default_name': self.summary,
-            'default_description': self.note and tools.html2plaintext(self.note) or '',
+            'default_description': self.note and tools.html2plaintext(self.note).strip() or '',
             'default_activity_ids': [(6, 0, self.ids)],
         }
         return action