Steps to reproduce:
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1. Create a first FSM task with an assignee (e.g. Mitchel Admin) and a planned date
2. Create a second FSM task with another assignee (e.g. Marc Demo) with the same planned date as in the first task
3. Create a third FSM task with both assignees (e.g. Mitchel Admin and Marc Demo) with again, the same planned date
4. Save the third task or change anything in it
5. A traceback occurs

Fix:
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In the computed method "_compute_planning_overlap()", the "overlap_mapping" dictionary is iteratively updated with the data of the tasks of the users which overlap with the current one.
When there were multiple assignees, it was possible that overlap_mapping[task_id] did not contain the user_id key of the assignee yet, hence the KeyError. To prevent that error, we check whether the user_id key exists and if not, we set it to an empty dictionary that will be filled in afterwards.

Tests:
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To be consistent with tests from TestPlanningOverlap in project_enterprise, the same test suite was reproduced for FSM tasks in TestFsmPlanningOverlap.

task-3812556
version-17.1