**Issue:**
Multiple errors occur when processing payroll for "Fully Flexible" employees
and overlapping leave scenarios:

1. ValueError "Expected singleton: hr.work.entry.type(7, 8)" during work entry generation
   when sick leave overlaps with public holiday

**Steps to Reproduce:**
1. Go to the **Employees** app and create a new employee.
   * Set the working hours to **empty (fully flexible)**.
2. Go to **Contracts** and create a new contract.
   * Set the **Work Entry Source** to *Attendance*.
   * Save and make the contract **Running**.
3. Go to **Time Off** → **New**, and create a sick time off for the employee.
   * Example: from **25th to 29th**.
   * Approve the time off.
4. Go to **Configuration** → **Public Holidays**, and create a new public holiday.
   * Example: **27th**, which overlaps with the sick time off.
   * Work Entry Type = **Paid Time Off**.
5. Go to **Payroll** → **Work Entries**.
   * A **traceback** occurs.

**Root Causes:**
- In `_get_interval_leave_work_entry_type()`: Direct access to `interval[2].work_entry_type_id.code` causes singleton violation when overlapping leaves create intervals containing multiple work entry types.

**Fix:**
- Replace direct access to `interval[2].work_entry_type_id.code` with safe recordset slicing `interval[2].work_entry_type_id[:1].code` to prevent singleton violation when interval contains multiple work entry types

This resolves payroll blocking issue for deployments using the
Fully Flexible employee feature, where employees may have
overlapping leave types and no assigned working calendar.

Test : [PR](https://github.com/odoo/enterprise/pull/93902)

opw-4979974

Forward-Port-Of: odoo/odoo#230659
Forward-Port-Of: odoo/odoo#223448