Improve `pretty_print`

### Privileged issue - [x] I am a LangChain maintainer, or was asked directly by a LangChain maintainer to create an issue here. ### Issue Content ## Description `pretty_print()` does not format non-string message content properly. When `AIMessage.content` is a list of content block dictionaries (rather than a plain string), the method simply dumps the raw Python representation of the list, making the output difficult to read. ## Current Behavior When a message contains structured content blocks (e.g., server-side tool calls, tool results, text with citations), `pretty_print()` outputs the raw dictionary representation: ```python ================================== Ai Message ================================== [{'text': "I'll search for information...", 'type': 'text'}, {'id': 'toolu_123', 'input': {'query': 'search query'}, 'name': 'web_search', 'type': 'server_tool_use'}, {'content': [{'encrypted_content': 'EoAlCioIChgC...', 'title': 'Page Title', 'type': 'web_search_result', 'url': 'https://example.com'}], 'tool_use_id': 'toolu_123', 'type': 'web_search_tool_result'}, {'citations': [{'cited_text': '...', 'encrypted_index': '...', 'title': '...', 'type': 'web_search_result_location', 'url': '...'}], 'text': 'Here is the answer...', 'type': 'text'}] ``` ## Expected Behavior Content blocks should be formatted in a human-readable way: ``` ================================== Ai Message ================================== I'll search for information... Server Tool Use: web_search (toolu_123) Args: query: search query [Web Search Results] - Page Title (https://example.com) Here is the answer... ``` ## Root Cause In `langchain_core/messages/base.py`, the `pretty_repr` method has a TODO comment acknowledging this limitation: ```python def pretty_repr(self, html: bool = False) -> str: title = get_msg_title_repr(self.type.title() + " Message", bold=html) # TODO: handle non-string content. if self.name is not None: title += f"\nName: {self.name}" …