FlareChain.from_llm ignores supplied llm

### Checked other resources

- [x] This is a bug, not a usage question. For questions, please use the LangChain Forum (https://forum.langchain.com/).
- [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] I read what a minimal reproducible example is (https://stackoverflow.com/help/minimal-reproducible-example).
- [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

### Example Code

Included here is a copy of the from_llm method from langchain.chains.flare.base. From this code example it is clear to see the llm the user pass gets overridden. 

```python
# Source excerpt from langchain.chains.flare.base (current main)
    @classmethod
    def from_llm(
        cls,
        llm: BaseLanguageModel,
        max_generation_len: int = 32,
        **kwargs: Any,
    ) -> FlareChain:
        """Creates a FlareChain from a language model.

        Args:
            llm: Language model to use.
            max_generation_len: Maximum length of the generated response.
            kwargs: Additional arguments to pass to the constructor.

        Returns:
            FlareChain class with the given language model.
        """
        try:
            from langchain_openai import ChatOpenAI
        except ImportError as e:
            msg = (
                "OpenAI is required for FlareChain. "
                "Please install langchain-openai."
                "pip install langchain-openai"
            )
            raise ImportError(msg) from e
        llm = ChatOpenAI(
            max_completion_tokens=max_generation_len,
            logprobs=True,
            temperature=0,
        )
```

### Error Message and Stack Trace (if applicable)

_No response_

### Description

Description: Calling FlareChain.from_llm with a user‑constructed BaseLanguageModel instance does not use that instance. The method unconditionally creates a new ChatOpenAI (temperature=0, its own max_completion_tokens) and overwrites the passed llm variable.

Expected: The provided llm instance (with its custom configuration, e.g. temperature=0.55, max_completion_tokens=17) is used to build response_chain and question_generator_chain.

Actual: A new ChatOpenAI(logprobs=True, temperature=0, max_completion_tokens=<max_generation_len>) is created inside from_llm, replacing the supplied object; the original instance is absent from the constructed chain graph.

Impact:

Prevents passing a preconfigured / alternative-compatible model via this API.
Signature and docstring are misleading (argument is ignored).
Users cannot tune temperature or other parameters through their own instance here.
Proposed Fix: Stop overwriting the passed llm; validate it is a ChatOpenAI with logprobs enabled, then compose the pipelines with that instance (optional: only construct a default if llm is None).

preposed fix #32847 

### System Info

System Information
------------------
> OS:  Darwin
> OS Version:  Darwin Kernel Version 24.0.0: Tue Sep 24 23:39:07 PDT 2024; root:xnu-11215.1.12~1/RELEASE_ARM64_T6000
> Python Version:  3.11.9 (main, May  4 2025, 14:33:11) [Clang 16.0.0 (clang-1600.0.26.3)]

Package Information
-------------------
> langchain_core: 0.3.75
> langchain: 0.3.27
> langsmith: 0.4.26
> langchain_openai: 0.3.32
> langchain_text_splitters: 0.3.11

Optional packages not installed
-------------------------------
> langserve

Other Dependencies
------------------
> async-timeout<5.0.0,>=4.0.0;: Installed. No version info available.
> httpx<1,>=0.23.0: Installed. No version info available.
> jsonpatch<2.0,>=1.33: Installed. No version info available.
> langchain-anthropic;: Installed. No version info available.
> langchain-aws;: Installed. No version info available.
> langchain-azure-ai;: Installed. No version info available.
> langchain-cohere;: Installed. No version info available.
> langchain-community;: Installed. No version info available.
> langchain-core<1.0.0,>=0.3.72: Installed. No version info available.
> langchain-core<1.0.0,>=0.3.74: Installed. No version info available.
> langchain-core<2.0.0,>=0.3.75: Installed. No version info available.
> langchain-deepseek;: Installed. No version info available.
> langchain-fireworks;: Installed. No version info available.
> langchain-google-genai;: Installed. No version info available.
> langchain-google-vertexai;: Installed. No version info available.
> langchain-groq;: Installed. No version info available.
> langchain-huggingface;: Installed. No version info available.
> langchain-mistralai;: Installed. No version info available.
> langchain-ollama;: Installed. No version info available.
> langchain-openai;: Installed. No version info available.
> langchain-perplexity;: Installed. No version info available.
> langchain-text-splitters<1.0.0,>=0.3.9: Installed. No version info available.
> langchain-together;: Installed. No version info available.
> langchain-xai;: Installed. No version info available.
> langsmith-pyo3>=0.1.0rc2;: Installed. No version info available.
> langsmith>=0.1.17: Installed. No version info available.
> langsmith>=0.3.45: Installed. No version info available.
> openai-agents>=0.0.3;: Installed. No version info available.
> openai<2.0.0,>=1.99.9: Installed. No version info available.
> opentelemetry-api>=1.30.0;: Installed. No version info available.
> opentelemetry-exporter-otlp-proto-http>=1.30.0;: Installed. No version info available.
> opentelemetry-sdk>=1.30.0;: Installed. No version info available.
> orjson>=3.9.14;: Installed. No version info available.
> packaging>=23.2: Installed. No version info available.
> pydantic<3,>=1: Installed. No version info available.
> pydantic<3.0.0,>=2.7.4: Installed. No version info available.
> pydantic>=2.7.4: Installed. No version info available.
> pytest>=7.0.0;: Installed. No version info available.
> PyYAML>=5.3: Installed. No version info available.
> requests-toolbelt>=1.0.0: Installed. No version info available.
> requests<3,>=2: Installed. No version info available.
> requests>=2.0.0: Installed. No version info available.
> rich>=13.9.4;: Installed. No version info available.
> SQLAlchemy<3,>=1.4: Installed. No version info available.
> tenacity!=8.4.0,<10.0.0,>=8.1.0: Installed. No version info available.
> tiktoken<1,>=0.7: Installed. No version info available.
> typing-extensions>=4.7: Installed. No version info available.
> vcrpy>=7.0.0;: Installed. No version info available.
> zstandard>=0.23.0: Installed. No version info available.

Labels: bug
State: closed