fix(client): handle legacy psr-18 client compatibility

- modify client instance checks in `Configuration.php` to handle psr-18
  clients correctly
- update `testWorksWithPsr18Client` to wrap client with required factories
- add new test `testWorksWithLegacyPsr18Client` for backward compatibility
- simplify array key check using `isset` instead of `array_key_exists`

Author: tharropoulos

src/Lib/Configuration.php

@@ -104,10 +104,10 @@ public function __construct(array $config)
         $this->logger   = new Logger('typesense');
         $this->logger->pushHandler(new StreamHandler('php://stdout', $this->logLevel));
 
-        if (true === \array_key_exists('client', $config)) {
-            if ($config['client'] instanceof HttpMethodsClient) {
+        if (isset($config['client'])) {
+            if ($config['client'] instanceof HttpMethodsClient || $config['client'] instanceof ClientInterface) {
                 $this->client = $config['client'];
-            } elseif ($config['client'] instanceof HttpClient || $config['client'] instanceof ClientInterface) {
+            } elseif ($config['client'] instanceof HttpClient) {
                 $this->client = new HttpMethodsClient(
                     $config['client'],
                     Psr17FactoryDiscovery::findRequestFactory(),

tests/Feature/HttpClientsTest.php

@@ -38,8 +38,13 @@ public function testWorksWithDefaultClient(): void
     public function testWorksWithPsr18Client(): void
     {
         $httpClient = new Psr18Client();
-        $config = array_merge($this->baseConfig, ['client' => $httpClient]);
+        $wrappedClient = new HttpMethodsClient(
+            $httpClient,
+            Psr17FactoryDiscovery::findRequestFactory(),
+            Psr17FactoryDiscovery::findStreamFactory()
+        );
 
+        $config = array_merge($this->baseConfig, ['client' => $wrappedClient]);
         $client = new Client($config);
         $response = $client->health->retrieve();
         $this->assertIsBool($response['ok']);
@@ -60,6 +65,14 @@ public function testWorksWithHttpMethodsClient(): void
         $this->assertIsBool($response['ok']);
     }
 
+    public function testWorksWithLegacyPsr18Client(): void
+    {
+        $httpClient = $this->createMock(\Psr\Http\Client\ClientInterface::class);
+        $config = array_merge($this->baseConfig, ['client' => $httpClient]);
+        $client = new Client($config);
+        $this->assertInstanceOf(Client::class, $client);
+    }
+
     public function testRejectsInvalidClient(): void
     {
         $this->expectException(ConfigError::class);