- Compute the standard durations for all French leaves in a single
  `_get_durations()` call instead of recomputing them for each leave.

- Reuse the precomputed duration mapping while calculating the French
  legal duration for each leave.

- Keep the existing French-specific duration calculation unchanged,
  including public holidays, company calendar, date extension, and
  half-day handling.

This avoids repeated ORM computations when processing multiple French
leaves in a batch and significantly improves the performance of leave
duration computation.

Performance testing was performed using the `hr.leave.employee.report`
introduced in 19.1, which calls `_get_durations()` on a large batch of
virtual leaves.

Performance testing on a database containing 677 leaves, including
298 French leaves:

| Metric                              | Before          | After       |
|------------------------------------|-----------------|-------------|
| Standard duration computations    | 298             | 1           |
| Total `_get_durations()`          | >169s (timeout) | ~24s        |

not specific to the employee report and benefits
any caller that invokes _get_durations() on a larger batch.

**opw-6421323**
---
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Forward-Port-Of: odoo/odoo#281165