refactor: Add index field to DuckDbMessageModel in monitor schema

This commit adds the `index` field to the `DuckDbMessageModel` class in the `monitor/schema.py` file. The `index` field is of type `int` and is used to store the index value for the message. This modification enhances the schema of the `DuckDbMessageModel` and allows for more efficient querying and sorting of messages based on their index.

src/backend/base/langflow/services/monitor/schema.py

@@ -82,6 +82,7 @@ class TransactionModelResponse(DefaultModel):
 
 
 class DuckDbMessageModel(DefaultModel):
+    index: int
     flow_id: str | None = Field(default=None, alias="flow_id")
     timestamp: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
     sender: str