I have to disagree with @ronroyston here. The OSI model is an idealized abstract model, and there are no protocols in use today that follow it. HTTP was created without regard to the OSI model, so there's no point to trying to make it fit. The TCP model is a little closer, because it lumps everything above transport into "application."
As the Wikipedia article says,
An HTTP session is a sequence of network request-response
transactions. An HTTP client initiates a request by establishing a
Transmission Control Protocol (TCP) connection to a particular port on
So a HTTP session is created by establishing a TCP connection. It follows that the session ends when the TCP connection is terminated.
Ultimately, many definitions like this are a bit squishy, so they can mean what you want them to mean. Just don't get too hung up on figuring out the OSI model. It's just a model.