v0.37.3 HTTP polling with structured auth

Fixed

• HTTP polling clients now read the structured auth rejection instead of discarding it. All three (HttpPollingConnection, BestHttpPollingConnection, UnityHttpConnection) threw on any non-2xx before touching the response body, so the SessionConnectResponse that 0.37.2 attached to the 401 never reached the dispatcher — reconnect recovery still worked, but only through the message-text fallback. A 401 on /session-connect is now parsed as an answer; every other status keeps throwing as before.
• A 401 from ASP.NET authorization middleware (.RequireAuthorization() on the endpoint) carries no body at all. Clients synthesize the same AuthenticationRequired answer for it, so a rejection at the middleware layer and one from the endpoint reach the dispatcher identically.

Added

• SessionConnectResponse.AuthenticationRequired(error) — one canonical construction of the rejection, now shared by both server transports and the three HTTP clients.