test(events): guard against double-registered transaction listeners

The PHP-LARAVEL-3V fix removed the explicit Event::listen calls that,
combined with event discovery, registered each transaction listener
twice. Add a regression test that fires each transaction event and
asserts its queued listener is dispatched exactly once, so re-adding an
explicit registration (or otherwise double-wiring) fails CI.

tests/Feature/Events/TransactionListenerRegistrationTest.php

@@ -0,0 +1,33 @@
+<?php
+
+use App\Events\TransactionCreated;
+use App\Events\TransactionDeleted;
+use App\Events\TransactionUpdated;
+use App\Listeners\AssignTransactionToBudget;
+use App\Listeners\UnassignTransactionFromBudget;
+use App\Models\Transaction;
+use App\Models\User;
+use Illuminate\Events\CallQueuedListener;
+use Illuminate\Support\Facades\Queue;
+
+function queuedListenerCount(string $listener): int
+{
+    return Queue::pushed(CallQueuedListener::class)
+        ->filter(fn (CallQueuedListener $job): bool => $job->class === $listener)
+        ->count();
+}
+
+it('registers each transaction listener exactly once', function (string $event, string $listener): void {
+    $user = User::factory()->onboarded()->create();
+    $transaction = Transaction::factory()->plaintext()->create(['user_id' => $user->id]);
+
+    Queue::fake();
+
+    event(new $event($transaction));
+
+    expect(queuedListenerCount($listener))->toBe(1);
+})->with([
+    'created dispatches budget assignment once' => [TransactionCreated::class, AssignTransactionToBudget::class],
+    'updated dispatches budget assignment once' => [TransactionUpdated::class, AssignTransactionToBudget::class],
+    'deleted dispatches budget unassignment once' => [TransactionDeleted::class, UnassignTransactionFromBudget::class],
+]);