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>

tests/Service/Contracts/EmployeeContractPhaseResolverTest.php

@@ -32,6 +32,7 @@ final class EmployeeContractPhaseResolverTest extends TestCase
         self::assertSame(ContractType::H39, $phases[0]->contractType);
         self::assertTrue($phases[0]->isCurrent);
         self::assertNull($phases[0]->endDate);
+        self::assertSame(ContractNature::CDI, $phases[0]->contractNature);
     }
 
     public function testThreeConsecutivePeriodsSameSignatureCollapseIntoSinglePhase(): void