feat(ollama): support response_format (#34612)

Fixes #34610

---

This PR resolves an issue where `ChatOllama` would raise an `unexpected
keyword argument 'response_format'` error when used with `create_agent`
or when passed an OpenAI-style `response_format`.

When using `create_agent` (especially with models like `gpt-oss`),
LangChain creates a `response_format` argument (e.g., `{"type":
"json_schema", ...}`). `ChatOllama` previously passed this argument
directly to the underlying Ollama client, which does not support
`response_format` and instead expects a `format` parameter.

## The Fix
I updated `_chat_params` in
`libs/partners/ollama/langchain_ollama/chat_models.py` to:
1.  Intercept the `response_format` argument.
2.  Map it to the native Ollama `format` parameter:
* `{"type": "json_schema", "json_schema": {"schema": ...}}` ->
`format=schema`
    *   `{"type": "json_object"}` -> `format="json"`
3.  Remove `response_format` from the kwargs passed to the client.

## Validation
* **Reproduction Script**: Verified the fix with a script covering
`json_schema`, `json_object`, and explicit `format` priority scenarios.
* **New Tests**: Added 3 new unit tests to
`libs/partners/ollama/tests/unit_tests/test_chat_models.py` covering
these scenarios.
* **Regression**: Ran the full test suite (`make -C libs/partners/ollama
test`), passing 29 tests (previously 26).
* **Lint/Format**: Verified with `make lint_package` and `make format`.

---------

Co-authored-by: Mohan Kumar Sagadevan <mohankumarsagadevan@Mohans-MacBook-Air.local>
Co-authored-by: Mason Daugherty <mason@langchain.dev>
Co-authored-by: Mason Daugherty <github@mdrxy.com>

libs/partners/ollama/langchain_ollama/chat_models.py

@@ -792,12 +792,17 @@ class ChatOllama(BaseChatModel):
                 if v is not None
             }
 
+        format_param = self._resolve_format_param(
+            kwargs.pop("format", self.format),
+            kwargs.pop("response_format", None),
+        )
+
         params = {
             "messages": ollama_messages,
             "stream": kwargs.pop("stream", True),
             "model": kwargs.pop("model", self.model),
             "think": kwargs.pop("reasoning", self.reasoning),
-            "format": kwargs.pop("format", self.format),
+            "format": format_param,
             "logprobs": kwargs.pop("logprobs", self.logprobs),
             "top_logprobs": kwargs.pop("top_logprobs", self.top_logprobs),
             "options": options_dict,
@@ -815,6 +820,107 @@ class ChatOllama(BaseChatModel):
 
         return params
 
+    def _resolve_format_param(
+        self,
+        format_param: str | dict[str, Any] | None,
+        response_format: Any | None,
+    ) -> str | dict[str, Any] | None:
+        """Resolve the format parameter.
+
+        Converts an OpenAI-style `response_format` dict to the `format`
+        parameter expected by Ollama.
+
+        Args:
+            format_param: The explicit `format` value (takes priority).
+            response_format: An OpenAI-style `response_format` dict.
+
+        Returns:
+            The resolved format value to pass to the Ollama client.
+        """
+        if format_param is not None:
+            if response_format is not None:
+                warnings.warn(
+                    "Both 'format' and 'response_format' were provided. "
+                    "'response_format' will be ignored in favor of 'format'.",
+                    UserWarning,
+                    stacklevel=2,
+                )
+            return format_param
+
+        if response_format is None:
+            return None
+
+        return self._convert_response_format(response_format)
+
+    def _convert_response_format(
+        self,
+        response_format: Any,
+    ) -> str | dict[str, Any] | None:
+        """Convert an OpenAI-style `response_format` to an Ollama `format` value.
+
+        Args:
+            response_format: The `response_format` value to convert.
+
+        Returns:
+            The Ollama-compatible `format` value, or `None` if conversion fails.
+        """
+        if not isinstance(response_format, dict):
+            warnings.warn(
+                f"Ignored invalid 'response_format' type: {type(response_format)}. "
+                "Expected a dictionary.",
+                UserWarning,
+                stacklevel=2,
+            )
+            return None
+
+        fmt_type = response_format.get("type")
+        if fmt_type == "json_object":
+            return "json"
+        if fmt_type == "json_schema":
+            return self._extract_json_schema(response_format)
+
+        warnings.warn(
+            f"Ignored unrecognized 'response_format' type: {fmt_type}. "
+            "Expected 'json_object' or 'json_schema'.",
+            UserWarning,
+            stacklevel=2,
+        )
+        return None
+
+    def _extract_json_schema(
+        self,
+        response_format: dict[str, Any],
+    ) -> dict[str, Any] | None:
+        """Extract the raw JSON schema from an OpenAI ``json_schema`` envelope.
+
+        Args:
+            response_format: A dict with ``type: "json_schema"``.
+
+        Returns:
+            The raw JSON schema dict, or ``None`` if extraction fails.
+        """
+        json_schema_block = response_format.get("json_schema")
+        if not isinstance(json_schema_block, dict):
+            warnings.warn(
+                "response_format has type 'json_schema' but 'json_schema' "
+                f"value is {type(json_schema_block)}, expected a dict "
+                "containing a 'schema' key. "
+                "The format parameter will not be set.",
+                UserWarning,
+                stacklevel=2,
+            )
+            return None
+        schema = json_schema_block.get("schema")
+        if schema is None:
+            warnings.warn(
+                "response_format has type 'json_schema' but no 'schema' "
+                "key was found in 'json_schema'. "
+                "The format parameter will not be set.",
+                UserWarning,
+                stacklevel=2,
+            )
+        return schema
+
     @model_validator(mode="after")
     def _set_clients(self) -> Self:
         """Set clients to use for ollama."""