pretty_repr(html=True) does not always return HTML-formatted output for non-string 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 - [x] 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-perplexity - [ ] langchain-prompty - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Reproduction Steps / Example Code (Python) ```python from langchain_core.messages import BaseMessage message = BaseMessage( content=["First", "Second", "Third"], type="ai", name="bot" ) text = message.text() print("text():", text) html_repr = message.pretty_repr(html=True) print("pretty_repr(html=True):", html_repr) ``` ### Error Message and Stack Trace (if applicable) ```shell text(): FirstSecondThird pretty_repr(html=True): ================================== Ai Message ================================== Name: bot ['First', 'Second', 'Third'] ``` ### Description I am using `BaseMessage.pretty_repr(html=True)` and expect the output to be HTML-formatted, as stated in the docstring: > *If `True`, the message will be formatted with HTML tags.* However, when `content` is provided as a non-string type (e.g. `list[str]`, which is a valid …