Harden the HTTP/2 connection-waiter registry against waiter failures and shutdown races

[hyperxpro]

Motivation:

#2227 introduced an event-driven HTTP/2 connection-waiter registry in ChannelManager, but three issues remained. A waiter throwing an exception could prevent subsequent waiters from being notified and interrupt the connection establishment path, leaking a permit and leaving the request hanging. Requests registering during shutdown could miss both the close notification and timeout, leaving their futures incomplete. Additionally, timed-out or removed waiters left empty per-host sets behind for the lifetime of the client.

Modification:

Isolate waiter callbacks in a notifyHttp2ConnectionWaiter try/catch helper. Add a waitersClosed flag that is latched before the shutdown sweep so new waiters are rejected during shutdown, allowing arm() to fail the request immediately without interacting with the timer. Remove empty waiter sets using computeIfPresent. Added four unit tests covering these scenarios.

Result:

Misbehaving waiters can no longer disrupt connection establishment or leak permits, waiters registered during shutdown fail deterministically instead of hanging, and the registry no longer retains empty per-host entries.