Issue #3478167: Add a global JSON decoder for both streaming and none streaming responses

I removed the implementation from it in the submodules, since that would just add a lot of files that are less important for the code review.

src/OperationType/Chat/ReplayedChatMessageIterator.php

+<?php
+
+namespace Drupal\ai\OperationType\Chat;
+
+/**
+ * Assistant Replay Stream message iterator.
+ */
+class ReplayedChatMessageIterator extends StreamedChatMessageIterator {
+
+  /**
+   * The first message to append.
+   */
+  private string $firstMessage;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIterator(): \Generator {
+    $i = 0;
+    foreach ($this->iterator->getIterator() as $data) {
+      $text = $i ? $data->getText() : $this->firstMessage . $data->getText();
+      $i++;
+      yield new StreamedChatMessage(
+        $data->getRole() ?? '',
+        $text ?? '',
+        $data->getMetadata() ?? []
+      );
+    }
+  }
+
+  /**
+   * Set the first message.
+   *
+   * @param string $message
+   *   The message.
+   */
+  public function setFirstMessage(string $message) {
+    $this->firstMessage = $message;
+  }
+
+}

[Marcus Johansson]

This is a mock iterator file, since we use simple generators they are consumed on usage, so we need to "virtually" replay them by adding the text we picked out so far.