Defer connectionEstablished observer callback and H2 auto-read until

[daschl]

after connection acceptors complete

Motivation

Two related issues stem from the same root cause — actions that should happen after all connection acceptors complete instead happen when the connection Single resolves in tryCompleteSubscriber/completeSubscriber:

1. connectionEstablished/multiplexedConnectionEstablished observer callbacks fire before Late/Legacy ConnectionAcceptors evaluate. This means connection time measurement excludes acceptor time, and the callback fires even if the acceptor later rejects the connection.

2. HTTP/2 server enables auto-read in AbstractH2ParentConnection#handlerAdded() before acceptors evaluate. If the client's HTTP/2 preface and request arrive in the same TCP segment, the server processes the request before the acceptor can reject — meaning StreamingHttpService.handle() gets invoked for connections that should have been rejected.

Modifications

1. Add a deferConnectionEstablished parameter to DefaultNettyConnection.initChannel() and guard the connectionEstablished call in completeSubscriber(). Add a method for callers to invoke after acceptors pass.

2. Add a deferAutoRead parameter to AbstractH2ParentConnection. When true, handlerAdded() no longer enables auto-read. The server passes true; the client passes false (unchanged behavior).

3. Add new tests covering observer callback timing and H2 auto-read deferral across H1, H2 prior-knowledge, and H2 ALPN code paths.

Result

connectionEstablished/multiplexedConnectionEstablished observer
callbacks now fire after all connection acceptors complete, and only
when the connection is accepted. HTTP/2 auto-read is deferred until
the connectionConsumer runs, preventing requests from reaching the
service handler before acceptors can reject. Client-side behavior is
unchanged.

[bryce-anderson]

It feels like the underlying problem is that we setup our pipelines too early. If that's true and how to address it might b a lot more work than we want to tackle.