Add chat history support

README.md

@@ -21,6 +21,7 @@ _This library is not developed or endorsed by Google._
   - [Basic text generation](#basic-text-generation)
   - [Multimodal input](#multimodal-input)
   - [Chat Session (Multi-Turn Conversations)](#chat-session-multi-turn-conversations)
+  - [Chat Session with history](#chat-session-with-history)
   - [Tokens counting](#tokens-counting)
   - [Listing models](#listing-models)
 
@@ -111,6 +112,44 @@ func main() {
 This code will print "Hello World!" to the standard output.
 ```
 
+### Chat Session with history
+
+```php
+$client = new GeminiAPI\Client('GEMINI_API_KEY');
+
+$history = [
+    Content::text('Hello World in PHP', Role::User),
+    Content::text(
+        <<<TEXT
+        <?php
+        echo "Hello World!";
+        ?>
+
+        This code will print "Hello World!" to the standard output.
+        TEXT,
+        Role::Model,
+    ),
+];
+$chat = $client->geminiPro()
+    ->startChat()
+    ->withHistory($history);
+
+$response = $chat->sendMessage(new TextPart('in Go'));
+print $response->text();
+```
+
+```text
+package main
+
+import "fmt"
+
+func main() {
+    fmt.Println("Hello World!")
+}
+
+This code will print "Hello World!" to the standard output.
+```
+
 ### Tokens counting
 
 ```php

src/ChatSession.php

@@ -8,10 +8,14 @@
 use GeminiAPI\Resources\Content;
 use GeminiAPI\Resources\Parts\PartInterface;
 use GeminiAPI\Responses\GenerateContentResponse;
+use GeminiAPI\Traits\ArrayTypeValidator;
+use InvalidArgumentException;
 use Psr\Http\Client\ClientExceptionInterface;
 
 class ChatSession
 {
+    use ArrayTypeValidator;
+
     /** @var Content[] */
     private array $history;
 
@@ -40,4 +44,27 @@ public function sendMessage(PartInterface ...$parts): GenerateContentResponse
 
         return $response;
     }
+
+    /**
+     * @return Content[]
+     */
+    public function history(): array
+    {
+        return $this->history;
+    }
+
+    /**
+     * @param Content[] $history
+     * @return $this
+     * @throws InvalidArgumentException
+     */
+    public function withHistory(array $history): self
+    {
+        $this->ensureArrayOfType($history, Content::class);
+
+        $clone = clone $this;
+        $clone->history = $history;
+
+        return $clone;
+    }
 }