Improve BaseMessage.pretty_repr for multimodal content

### Checked other resources - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs _No response_ ### Reproduction Steps / Example Code (Python) ```python from langchain_core.messages import HumanMessage msg = HumanMessage(content=[ {"type": "text", "text": "What is in this image?"}, {"type": "image_url", "image_url": {"url": "https://example.com/image.png"}} ]) # Currently outputs raw Python dictionaries print("Current Output:") msg.pretty_print() ``` ### Error Message and Stack Trace (if applicable) ```shell ``` ### Description Currently, the `pretty_repr` method in `BaseMessage` (used for `pretty_print()`) only handles string content fully. When a message contains a list of multimodal content blocks (e.g., text and image_url), it defaults to printing the raw list and dictionaries, which is difficult for developers to read during debugging. I've already developed a fix that adds human-readable…