Drop unique constraint on 'cif' in debitures table

Remove the unique constraint on the 'cif' field in the debitures table and refactor the `UniqueCifExceptZero` validation rule to handle the uniqueness check more robustly. Also, add an unused import statement for `UniqueExcept` in `DebitureRequest`.

database/migrations/2024_11_01_023117_update_debitures_table.php

@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('debitures', function (Blueprint $table) {
+            $table->dropUnique(['cif']);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('debitures', function (Blueprint $table) {
+            $table->string('cif')->unique()->change();
+        });
+    }
+};