Add extra_body parameter support in openrouter (#3479)

Closes #3263

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Adds support for an extra_body parameter in ChatOpenRouter to pass
custom request fields to OpenRouter API calls. This adds flexibility for
provider-specific options and fulfills Linear #3263.

- **New Features**
- Added extra_body field to ChatOpenRouter and spread it into ainvoke
requests.
  - Default is None, so no breaking changes.

<sup>Written for commit 7e477ea. Summary will update automatically on
new commits.</sup>

<!-- End of auto-generated description by cubic. -->

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browser_use/llm/openrouter/chat.py

@@ -48,6 +48,7 @@ class ChatOpenRouter(BaseChatModel):
 	default_query: Mapping[str, object] | None = None
 	http_client: httpx.AsyncClient | None = None
 	_strict_response_validation: bool = False
+	extra_body: dict[str, Any] | None = None
 
 	# Static
 	@property
@@ -148,6 +149,7 @@ class ChatOpenRouter(BaseChatModel):
 					top_p=self.top_p,
 					seed=self.seed,
 					extra_headers=extra_headers,
+					**(self.extra_body or {}),
 				)
 
 				usage = self._get_usage(response)
@@ -178,6 +180,7 @@ class ChatOpenRouter(BaseChatModel):
 						type='json_schema',
 					),
 					extra_headers=extra_headers,
+					**(self.extra_body or {}),
 				)
 
 				if response.choices[0].message.content is None: