fix: improve model availability checks

- Add has_model method to ModelsRoutingTable for checking pre-registered models
- Update check_model_availability to check model_store before provider APIs

llama_stack/core/routing_tables/models.py

@@ -67,6 +67,19 @@ class ModelsRoutingTable(CommonRoutingTableImpl, Models):
             raise ValueError(f"Provider {model.provider_id} not found in the routing table")
         return self.impls_by_provider_id[model.provider_id]
 
+    async def has_model(self, model_id: str) -> bool:
+        """
+        Check if a model exists in the routing table.
+
+        :param model_id: The model identifier to check
+        :return: True if the model exists, False otherwise
+        """
+        try:
+            await lookup_model(self, model_id)
+            return True
+        except ModelNotFoundError:
+            return False
+
     async def register_model(
         self,
         model_id: str,