add initial solution (made with copilot)

tests/Unit/RateLimiterTest.php

@@ -0,0 +1,36 @@
+<?php
+use PHPUnit\Framework\TestCase;
+use Core\RateLimiter\FileRateLimiter;
+
+final class RateLimiterTest extends TestCase {
+    private $dir;
+
+    protected function setUp(): void {
+        $this->dir = sys_get_temp_dir() . '/api_rate_limit_test';
+        if (is_dir($this->dir)) {
+            array_map('unlink', glob("$this->dir/*"));
+        } else {
+            mkdir($this->dir, 0700, true);
+        }
+    }
+
+    public function testAllowsRequestsUnderLimit(): void {
+        $limiter = new FileRateLimiter($this->dir);
+        $key = 'test-client';
+        $allowed = 0;
+        for ($i = 0; $i < 5; $i++) {
+            if ($limiter->allow($key)) $allowed++;
+        }
+        $this->assertGreaterThan(0, $allowed);
+    }
+
+    public function testBlocksWhenExceeded(): void {
+        $limiter = new FileRateLimiter($this->dir);
+        $key = 'test-client-2';
+        $requests = (int) \Core\Config::get('rate_limit_requests', 5);
+        for ($i = 0; $i < $requests; $i++) {
+            $this->assertTrue($limiter->allow($key));
+        }
+        $this->assertFalse($limiter->allow($key));
+    }
+}