perf: skip the WebSocket handshake probe without an Upgrade header

[lesnik512]

The middleware chose the child's scope by running web.WebSocketResponse().can_prepare(request) on every request. For a plain HTTP request that probe builds a WebSocketResponse, then builds, raises and catches an HTTPBadRequest, because the first check in aiohttp's _handshake is that Upgrade equals websocket.

The probe now runs only when the request has an Upgrade header. Without one the result was already "not a WebSocket", so the scope chosen for every request is the same as before. Any request that does carry the header is still decided by aiohttp's own check, as ADR-0002 requires. The ADR gets one sentence about the skip.

Measured by calling the middleware directly with a mocked GET request, median of 7 runs x 50k calls, run twice:

middleware	us per request
main	3.99
branch	1.30

The new test wraps web.WebSocketResponse in a mock and asserts that a plain GET never constructs one. It fails on main with assert 1 == 0. The existing WebSocket tests cover the upgrade path.