refactor(exercise) : extract ExerciseYearResolver to dedup year formula

Pull the "date -> leave/RTT exercise year" formula out of
EmployeeRttPaymentProcessor, EmployeeRttSummaryProvider and
EmployeeLeaveSummaryProvider into a single
App\Service\Exercise\ExerciseYearResolver. Forfait flag is parameterised
so the leave (calendar year) and RTT (Juin N-1 -> Mai N) variants share
the same implementation. Pure refactor, no behavioural change.

tests/State/EmployeeRttPaymentProcessorTest.php

@@ -10,6 +10,7 @@ use App\Entity\EmployeeContractPeriod;
 use App\Enum\ContractNature;
 use App\Enum\TrackingMode;
 use App\Service\Contracts\EmployeeContractPhaseResolver;
+use App\Service\Exercise\ExerciseYearResolver;
 use App\State\EmployeeRttPaymentProcessor;
 use DateTimeImmutable;
 use PHPUnit\Framework\TestCase;
@@ -141,6 +142,7 @@ final class EmployeeRttPaymentProcessorTest extends TestCase
 
         $this->setReadonlyProperty($processor, 'phaseResolver', new EmployeeContractPhaseResolver());
         $this->setReadonlyProperty($processor, 'clock', $clock);
+        $this->setReadonlyProperty($processor, 'exerciseYearResolver', new ExerciseYearResolver());
 
         return $processor;
     }