Feature: add support for reasoning content in streamed completions API

src/Responses/Chat/CreateStreamedResponseDelta.php

@@ -12,6 +12,7 @@ final class CreateStreamedResponseDelta
     private function __construct(
         public readonly ?string $role,
         public readonly ?string $content,
+        public readonly ?string $reasoningContent,  
         public readonly array $toolCalls,
         public readonly ?CreateStreamedResponseFunctionCall $functionCall,
     ) {}
@@ -28,6 +29,7 @@ public static function from(array $attributes): self
         return new self(
             $attributes['role'] ?? null,
             $attributes['content'] ?? null,
+            $attributes['reasoning_content'] ?? null,
             $toolCalls,
             isset($attributes['function_call']) ? CreateStreamedResponseFunctionCall::from($attributes['function_call']) : null,
         );

tests/Responses/Chat/CreateStreamedResponseDelta.php

@@ -18,6 +18,20 @@
     expect($result)
         ->role->toBeNull()
         ->content->toBe('Hello')
+        ->reasoningContent->toBeNull()
+        ->functionCall->toBeNull();
+});
+
+test('from reasoning content chunk', function () {
+    $delta = [
+        'reasoning_content' => 'Let me think about this...',
+    ];
+    $result = CreateStreamedResponseDelta::from($delta);
+
+    expect($result)
+        ->role->toBeNull()
+        ->content->toBeNull()
+        ->reasoningContent->toBe('Let me think about this...')
         ->functionCall->toBeNull();
 });