fix(ollama): avoid leading newline in multimodal text content (#37481)

Fixes #37480 

---

Avoid prepending a leading newline when converting multimodal message
text content in `ChatOllama`. Previously, the first text segment in
list-format message content was always prefixed with `\n`, which could
break formatting-sensitive vision/OCR models.

Also adds a regression test to ensure multimodal message content does
not start with an unintended leading newline.

Verified by running:

- make format
- make lint
- make test

libs/partners/ollama/langchain_ollama/chat_models.py

@@ -1024,9 +1024,13 @@ class ChatOllama(BaseChatModel):
             else:  # List
                 for content_part in message.content:
                     if isinstance(content_part, str):
-                        content += f"\n{content_part}"
+                        if content:
+                            content += "\n"
+                        content += content_part
                     elif content_part.get("type") == "text":
-                        content += f"\n{content_part['text']}"
+                        if content:
+                            content += "\n"
+                        content += content_part["text"]
                     elif content_part.get("type") == "tool_use":
                         continue
                     elif content_part.get("type") == "image_url":