refactor(booking): extract ServiceBookingCalculator from the controller

"Allowed duration of a service combination" lived inside
AppointmentController::serviceSlots(). Three upcoming callers need the same
computation (PATCH duration validation, service-aware reschedule, reserve
conversion); copying it would mean four variants with four different edge-case
behaviours.

The extraction is behaviour-preserving: BaseController::error() and
ExceptionSubscriber emit an identical envelope, so returning $this->error() was
replaced by throwing AppException with the same code/message/field.

Tenant ownership now goes through TenantOwnershipChecker::belongsToPair() (the
documented single point) instead of an inline section pair comparison. The repo
property is named itemRepo on purpose: TenantLookupInventoryTest only counts
recognised property names, so any other name would slip past the safety net.

The naive duration sum is kept deliberately — switching to solo/additional
minutes is task 04 and changes one line here.

Task: docs/new_feture/taskes/task-00-service-mode-completion/
Slot-mode contract: unchanged (--group=slot-mode-frozen green)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/Appointment/ValueObject/ServiceBookingDuration.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace App\Appointment\ValueObject;
+
+use App\ClinicService\Entity\ServiceItem;
+
+/**
+ * نتیجهٔ محاسبهٔ مدت یک ترکیب سرویس در حالت نوبت‌دهی سرویسی.
+ *
+ * `bufferMinutes` فاصلهٔ بین دو نوبت است و **جزو مدت نوبت نیست**: زمان پایانِ ذخیره‌شدهٔ
+ * نوبت `start + totalMinutes` است، نه `start + totalMinutes + buffer`. همان قاعده‌ای که
+ * {@see \App\Appointment\Service\SlotCalculatorService::getServiceStartTimes()} دارد.
+ */
+final readonly class ServiceBookingDuration
+{
+    /**
+     * @param ServiceItem[] $serviceItems به ترتیب uuidهای ورودی
+     * @param string[]      $warnings     پیام‌های فارسی برای نمایش؛ مانع عملیات نیستند
+     */
+    public function __construct(
+        public int $totalMinutes,
+        public int $bufferMinutes,
+        public array $serviceItems,
+        public array $warnings = [],
+    ) {}
+
+    /** زمان پایان نوبت برای یک زمان شروع مشخص — بدون بافر. */
+    public function endFor(int $slotStart): int
+    {
+        return $slotStart + $this->totalMinutes * 60;
+    }
+}