fix: wrap long line in chat_prompt_builder and add release notes

- Break line 272 in chat_prompt_builder.py to satisfy E501 (max 120 chars)
- Add reno release note for the dataclass mutation fix

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Aftabbs

haystack/components/builders/chat_prompt_builder.py

@@ -269,7 +269,9 @@ def run(
                     compiled_template = self._env.from_string(message.text)
                     rendered_text = compiled_template.render(template_variables_combined)
                     # use dataclasses.replace to avoid in-place mutation of the copied message
-                    rendered_message: ChatMessage = replace(deepcopy(message), _content=[TextContent(text=rendered_text)])
+                    rendered_message: ChatMessage = replace(
+                        deepcopy(message), _content=[TextContent(text=rendered_text)]
+                    )
                     processed_messages.append(rendered_message)
                 else:
                     processed_messages.append(message)

releasenotes/notes/fix-dataclass-inplace-mutation-3e8c1f92b4a07d56.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixed in-place mutation of ``ChatMessage`` dataclass instances in
+    ``ChatPromptBuilder``, ``DynamicChatPromptBuilder``, and
+    ``HuggingFaceLocalChatGenerator``. Mutations now use
+    ``dataclasses.replace()`` instead of direct attribute assignment,
+    preventing unintended side-effects when the same message object is
+    reused across multiple pipeline runs.