fix: correctly distinguish between first participant and applicant

src/Form/Service/Condition/FirstParticipantCondition.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Form\Service\Condition;
+
+use App\Form\Model\BookingDto;
+use App\Form\Service\Contract\FieldConditionInterface;
+
+/**
+ * Condition that checks if the participant is the first participant (index 0).
+ *
+ * This is used to apply field state logic specifically for the first participant
+ * in the booking, such as requiring address fields or showing DOB-dependent fields.
+ *
+ * Note: This checks if the participant is at index 0, NOT if they are the "applicant"
+ * in the BPN sense (the logged-in user). For agency-initiated bookings, the first
+ * participant may be different from the applicant/logged-in user.
+ */
+class FirstParticipantCondition implements FieldConditionInterface
+{
+    /**
+     * Evaluates if the participant is the first participant.
+     *
+     * @param BookingDto           $bookingDto       The current booking data
+     * @param int                  $participantIndex The index of the participant being evaluated
+     * @param array<string, mixed> $formData         Current form data (unused)
+     *
+     * @return bool True if the participant is the first participant (index 0)
+     */
+    public function evaluate(BookingDto $bookingDto, int $participantIndex, array $formData): bool
+    {
+        return 0 === $participantIndex;
+    }
+
+    public function getDependentFields(): array
+    {
+        return [];
+    }
+
+    public function getDescription(): string
+    {
+        return 'Checks if the participant is the first participant (index 0)';
+    }
+}