Unable to set reasoning for gpt-oss when using ChatOllama

### 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

I first installed `langchain==0.3.27` and `langchain_ollama==0.3.6`. Then I ran the following code:

```python
from langchain_ollama import ChatOllama

model = ChatOllama(model="gpt-oss:20b", reasoning="low")
```

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

```
Traceback (most recent call last):
  File "/.../bug.py", line 3, in <module>
    model = ChatOllama(model="gpt-oss:20b", reasoning="low")
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/.../.venv/lib/python3.12/site-packages/langchain_core/load/serializable.py", line 130, in __init__
    super().__init__(*args, **kwargs)
  File "/.../.venv/lib/python3.12/site-packages/pydantic/main.py", line 253, in __init__
    validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for ChatOllama
reasoning
  Input should be a valid boolean, unable to interpret input [type=bool_parsing, input_value='low', input_type=str]
    For further information visit https://errors.pydantic.dev/2.11/v/bool_parsing
```

### Description

Hi! I am trying to set the reasoning level to `"low"` for `gpt-oss:20b` when using `ChatOllama`. It appears that the reasoning level is currently required to be a boolean; as such, I am running into validation issues. However, `gpt-oss:20b` allows for setting the reasoning level to be `"low"/"medium"/"high"`. This is currently supported in the Ollama Python client with the `think` parameter, as shown here (to my understanding, `think` is equivalent to ChatOllama's `reasoning`): https://github.com/ollama/ollama-python/blob/main/examples/thinking-levels.py

Can `ChatOllama` be modified to allow string parameters for reasoning? I am happy to make this change, though I may need some guidance as a first-time contributor.

### System Info

```
System Information
------------------
> OS:  Linux
> OS Version:  #141~20.04.1-Ubuntu SMP Thu Jan 16 18:38:51 UTC 2025
> Python Version:  3.12.11 (main, Jul 23 2025, 00:34:44) [Clang 20.1.4 ]

Package Information
-------------------
> langchain_core: 0.3.74
> langchain: 0.3.27
> langsmith: 0.4.14
> langchain_ollama: 0.3.6
> langchain_text_splitters: 0.3.9

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,>=0.3.70: Installed. No version info available.
> langchain-core<1.0.0,>=0.3.72: 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.
> ollama<1.0.0,>=0.5.1: Installed. No version info available.
> openai-agents>=0.0.3;: 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.
> 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