Prevent mandatory jobs from being added to the rejected jobs

When user de-selects jobs in the UI, they may de-select mandatory jobs.
These jobs, even if de-selected, will still be added to the list of jobs
to run later on, and will be executed.

Therefore, they should not be part of the rejected jobs list.

checkbox-ng/plainbox/impl/session/assistant.py

@@ -1010,7 +1010,7 @@ def use_alternate_selection(self, selection: "Iterable[str]"):
         for job_id in self.get_static_todo_list():
             if job_id in selection:
                 desired_job_list.append(self._context.get_unit(job_id, "job"))
-            else:
+            elif job_id not in self.get_mandatory_jobs():
                 self._metadata.rejected_jobs.append(job_id)
         self._context.state.update_desired_job_list(desired_job_list)