Bug: ASGIConnection.session incorrect type - may return Empty

[peterschutt]

If setting scope["session"] to Empty is the same as the session not being set, then perhaps we should change ASGIConnection.session to raise if it is accessed when scope["session"] == Empty. At least that way we are sure to honor the type of the property. E.g.,:

    @property
    def session(self) -> dict[str, Any]:
        """Return the session for this connection if a session was previously set in the ``Scope``

        Returns:
            A dictionary representing the session value - if existing.

        Raises:
            ImproperlyConfiguredException: if session is not set in scope.
        """
        if "session" not in self.scope or self.scope["session"] is Empty:  # <---- add this "or" clause
            raise ImproperlyConfiguredException(
                "'session' is not defined in scope, install a SessionMiddleware to set it"
            )

        return cast("dict[str, Any]", self.scope["session"])

Originally posted by @peterschutt in #2427 (comment)

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[peterschutt]

The potential Empty value for connection.session comes from:

litestar/litestar/security/session_auth/middleware.py

Lines 116 to 119 in c28cde0

	if not connection.session or connection.scope["session"] is Empty:
	# the assignment of 'Empty' forces the session middleware to clear session data.
	connection.scope["session"] = Empty
	raise NotAuthorizedException("no session data found")