feat: add MessageBag::withMessageBag() method (#21)

Or what about naming it `merge()`?

Author: OskarStark

src/Message/MessageBag.php

@@ -33,6 +33,14 @@ public function with(Message $message): self
         return $messages;
     }
 
+    public function merge(MessageBag $messageBag): self
+    {
+        $messages = clone $this;
+        $messages->exchangeArray(array_merge($messages->getArrayCopy(), $messageBag->getArrayCopy()));
+
+        return $messages;
+    }
+
     public function withoutSystemMessage(): self
     {
         $messages = clone $this;

tests/Message/MessageBagTest.php

@@ -59,6 +59,23 @@ public function with(): void
         self::assertSame('It is time to wake up.', $newMessageBag[3]->content);
     }
 
+    #[Test]
+    public function merge(): void
+    {
+        $messageBag = new MessageBag(
+            Message::forSystem('My amazing system prompt.'),
+            Message::ofAssistant('It is time to sleep.'),
+            Message::ofUser('Hello, world!'),
+        );
+
+        $messageBag = $messageBag->merge(new MessageBag(
+            Message::ofAssistant('It is time to wake up.')
+        ));
+
+        self::assertCount(4, $messageBag);
+        self::assertSame('It is time to wake up.', $messageBag[3]->content);
+    }
+
     #[Test]
     public function withoutSystemMessage(): void
     {