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.

src/Service/Exercise/ExerciseYearResolver.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Service\Exercise;
+
+use DateTimeImmutable;
+
+final readonly class ExerciseYearResolver
+{
+    /**
+     * Convert a date to its leave/RTT exercise year.
+     *
+     * - Forfait: calendar year (Jan→Dec) — returns $date.Y.
+     * - Non-forfait: leave year (Juin N-1 → Mai N) — returns $date.Y+1 if month >= 6, else $date.Y.
+     */
+    public function forDate(DateTimeImmutable $date, bool $isForfait = false): int
+    {
+        if ($isForfait) {
+            return (int) $date->format('Y');
+        }
+
+        return (int) $date->format('n') >= 6
+            ? (int) $date->format('Y') + 1
+            : (int) $date->format('Y');
+    }
+}