[8.x] Add the Countable interface to AssertableJsonString (#34284)

* [8.x] Add the Countable interface to AssertableJsonString

This pr adds the Countable interface to the new `Illuminate\Testing\AssertableJsonString`. This adds downgrade compatibility to tests counting the decoded json response:

```php
$this->assertCount(1, $response->decodeJsonResponse());
```

* Apply fixes from StyleCI

src/Illuminate/Testing/AssertableJsonString.php

@@ -3,13 +3,14 @@
 namespace Illuminate\Testing;
 
 use ArrayAccess;
+use Countable;
 use Illuminate\Contracts\Support\Jsonable;
 use Illuminate\Support\Arr;
 use Illuminate\Support\Str;
 use Illuminate\Testing\Assert as PHPUnit;
 use JsonSerializable;
 
-class AssertableJsonString implements ArrayAccess
+class AssertableJsonString implements ArrayAccess, Countable
 {
     /**
      * The original encoded json.
@@ -329,6 +330,16 @@ protected function jsonSearchStrings($key, $value)
         ];
     }
 
+    /**
+     * Get the total number of items.
+     *
+     * @return int
+     */
+    public function count()
+    {
+        return count($this->decoded);
+    }
+
     /**
      * Determine whether an offset exists.
      *