fix: error in flowstate on single id & h of 1

[spolisar]

Fixes an error that occurred with flowstate when using data with a single id and an h of 1.

The error can be created with the following code.

import nest_asyncio
nest_asyncio.apply()

import pandas as pd

import timecopilot
from timecopilot.models.foundation.flowstate import FlowState

flowstate_model = FlowState()

forecaster = timecopilot.TimeCopilotForecaster(
    models=[
        flowstate_model
    ]
)

flowstate_df = df = pd.read_csv("https://timecopilot.s3.amazonaws.com/public/data/air_passengers.csv",
                                parse_dates=["ds"])
# flowstate_df = pd.read_csv(
#     "https://timecopilot.s3.amazonaws.com/public/data/the_anomaly_tour.csv",
#     parse_dates=["ds"],
# )


forecaster.forecast(
    df=flowstate_df,
    h=1
)