Steps to reproduce:
- Create one user linked to two companies.
- Create one employee per company for that user.
- Select one company and approve a time off for the employee.
- Open Tasks > Gantt and create a task during the approved time-off period the warning is shown and the Gantt cell is grayed out.
- Select both companies and create a task during the same time-off period in Tasks > Gantt.

Issue:
When multiple companies are selected, the time-off warning is still displayed but the corresponding Gantt cells are no longer grayed out, leading to an inconsistency between the warning logic and the Gantt rendering.

Cause:
In multi-company setups, a user can be linked to multiple resources. The Gantt unavailability logic assumed a one-to-one relationship between user and resource causing unavailability intervals from some resources to be overwritten.

Solution:
Aggregate unavailability intervals from all resources linked to the same user, limited to the selected companies, and merge them with the company calendar unavailability to ensure consistent Gantt gray rendering.
Related PR: https://github.com/odoo/enterprise/pull/57028

task-5089385

Forward-Port-Of: odoo/enterprise#105288

[FIX] mrp_workorder_plm: avoid singleton traceback when matching BOM operations
**Issue**
Adding a step in a manufacturing order with two operations that are too similar
could lead to a traceback when proposing an improvement from the Shop Floor.

**Steps to reproduce**
- Install the Product Lifecycle Management app.
- Create a BOM for any product with two operations that:
	- Have the same name and work center
	- Have no variant
- Create and confirm a Manufacturing Order for that product.
- Open the Shop Floor view.
- Click the gear icon -> Update Instructions -> Improvement Suggestion -> Add a Step -> Propose a Change.
-> A traceback occurs

**Cause**
When adding a step:
https://github.com/odoo/enterprise/blob/1c02615b73a2b4fb9f1ab91167a3c8d7c9ac824c/mrp_workorder_plm/models/mrp_workorder.py#L10
It tries to find the corresponding operation in the ECO's new BoM.
This relies on `_get_sync_values()`:
https://github.com/odoo/enterprise/blob/1c02615b73a2b4fb9f1ab91167a3c8d7c9ac824c/mrp_plm/models/mrp_routing.py#L9-L13
Because two operations share the same name, work center, and no variant, both match the filter:
https://github.com/odoo/enterprise/blob/1c02615b73a2b4fb9f1ab91167a3c8d7c9ac824c/mrp_workorder_plm/models/mrp_workorder.py#L39
This results in a singleton error when accessing `operation.id`:
https://github.com/odoo/enterprise/blob/1c02615b73a2b4fb9f1ab91167a3c8d7c9ac824c/mrp_workorder_plm/models/mrp_workorder.py#L42

opw-6241880

Forward-Port-Of: odoo/enterprise#119404