Handle partials in pretty_repr()

### Submission checklist - [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) - [x] 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-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs None found. ### Reproduction Steps / Example Code (Python) ```python p1 = PromptTemplate.from_template("Hello {user}, how is {input}?").partial(user="Lucy") print(p1.pretty_repr()) p2 = HumanMessagePromptTemplate.from_template( "Hello {user}, how is {input}?", partial_variables={"user": "Lucy"} ) print(p2.pretty_repr()) inner = PromptTemplate.from_template( "Hello {user}, how is {input}?", partial_variables={"user": "Lucy"} ) p3 = ChatMessagePromptTemplate(role="user", prompt=inner) print(p3.pretty_repr()) ``` ### Error Message and Stack Trace (if applicable) ```shell ``` ### Description Issue 1: PromptTemplate.pretty_repr() -> StringPromptTemplate Issue 2: HumanMessagePromptTemplate.pretty_repr() -> \_StringImageMessagePromptTemplate Issue 3: ChatMessagePromptTemplate.pretty_repr() -> BaseStringMessagePromp…