feat(fireworks,groq,openrouter): add standard model property (#35542)

Add a `model` property to `ChatFireworks`, `ChatGroq`, and
`ChatOpenRouter` that returns `model_name`. These partners use
Pydantic's `Field(alias="model")` on `model_name`, which means
`instance.model` doesn't work as a read accessor after construction — it
raises an `AttributeError` or returns the field descriptor. `ChatOpenAI`
already has this property; this brings the remaining in-repo partners to
parity.

libs/partners/fireworks/langchain_fireworks/chat_models.py

@@ -328,6 +328,11 @@ class ChatFireworks(BaseChatModel):
     model_name: str = Field(alias="model")
     """Model name to use."""
 
+    @property
+    def model(self) -> str:
+        """Same as model_name."""
+        return self.model_name
+
     temperature: float | None = None
     """What sampling temperature to use."""