refactor(leave) : address Task 3 review (helper, dead param, phase nature, regression test)

- Extract private helper `exerciseYearForDate(date, isForfait)` to dedupe
  the date->leave-exercise-year expression duplicated across `clampYearToPhase`
  and `resolveFirstComputationYear` (4 copies collapsed into 1 helper + 4
  call sites).

- Remove the unused `ContractPhase $phase` parameter from
  `resolveLeavePeriodBounds`: the body never reads it (the phase cap is
  applied later by `resolvePeriodBounds`).

- Add `ContractNature $contractNature` to `ContractPhase` DTO, populated
  from the first period of the group by `EmployeeContractPhaseResolver`.
  Drop the `resolveNatureForPhase` lookup in `EmployeeLeaveSummaryProvider`
  in favor of `$phase->contractNature`. Expose `contractNature` in
  `Employee::getContractPhases()` array shape for frontend use.

- Fix regression for terminated employees calling `computeYearSummary`
  without an explicit phase (LeaveRecapRowBuilder,
  DumpVerificationSnapshotCommand). Before the refactor the period bounds,
  accrual end and taken end were NOT capped at the contract end for
  terminated employees, because `Employee::getCurrentContractEndDate()`
  returns null when no period covers "today". The new fallback phase
  (`isCurrent=false`, real `endDate`) was silently capping `to`. Add an
  internal `applyPhaseEndCap` flag, true when phase is explicit, false
  for legacy callers, threaded through `resolvePeriodBounds`,
  `resolveAccrualCalculationEndDate` and `resolveTakenCalculationEndDate`.

- Add regression test
  `testTerminatedEmployeeWithoutExplicitPhaseSkipsPhaseEndCap` proving
  that legacy callers keep the natural exercise upper bound while explicit
  phase callers get the cap.

- Add `contractNature` assertion in `EmployeeContractPhaseResolverTest`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Entity/Employee.php

@@ -439,7 +439,8 @@ class Employee
      *   startDate: string,
      *   endDate: ?string,
      *   periodIds: list<int>,
-     *   isCurrent: bool
+     *   isCurrent: bool,
+     *   contractNature: string
      * }>
      */
     #[Groups(['employee:read'])]
@@ -449,14 +450,15 @@ class Employee
 
         return array_map(
             static fn (ContractPhase $phase): array => [
-                'id'           => $phase->id,
-                'contractType' => $phase->contractType->value,
-                'weeklyHours'  => $phase->weeklyHours,
-                'isDriver'     => $phase->isDriver,
-                'startDate'    => $phase->startDate->format('Y-m-d'),
-                'endDate'      => $phase->endDate?->format('Y-m-d'),
-                'periodIds'    => $phase->periodIds,
-                'isCurrent'    => $phase->isCurrent,
+                'id'             => $phase->id,
+                'contractType'   => $phase->contractType->value,
+                'weeklyHours'    => $phase->weeklyHours,
+                'isDriver'       => $phase->isDriver,
+                'startDate'      => $phase->startDate->format('Y-m-d'),
+                'endDate'        => $phase->endDate?->format('Y-m-d'),
+                'periodIds'      => $phase->periodIds,
+                'isCurrent'      => $phase->isCurrent,
+                'contractNature' => $phase->contractNature->value,
             ],
             $resolver->resolvePhases($this),
         );