proxy-io: Generalize ConnectStream / ServeStream for use in tests #365

Add an accessor and a Connections type alias for the EventLoop's
list of incoming connections, so future code can be simplified to
locate a specific connection without directly accessing the private
list or embedding a Connection object itself.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Add a _Serve/ServeStream overload accepting the init object as a
shared_ptr, so callers can transfer ownership instead of always
passing a reference to an object they keep alive themselves. Existing
reference-taking callers keep working through a thin overload that
wraps the reference in a shared_ptr with an empty deleter.

Also return the constructed ProxyServer along with an iterator to its
Connection in loop.m_incoming_connections, so callers can look up or
erase the connection later without embedding a Connection object
themselves.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Give ServeStream and ConnectStream a destroy_connection parameter,
defaulting to true, so callers can opt out of automatic connection
teardown and manage the Connection's lifetime themselves instead.
ServeStream gates the internal disconnect handler's list erase on the
parameter; ConnectStream just forwards it to the existing
ProxyClientBase parameter of the same name.

This lets callers that need to keep a connection alive past a
disconnect notification (e.g. to let in-flight server calls finish)
use these helpers instead of constructing a Connection manually.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Replace TestSetup's manual Connection construction with
ServeStream/ConnectStream, following the same pattern already used in
Bitcoin Core's own IPC test and fuzz code. This drops server_on_disconnect
entirely: ServeStream's destroy_connection parameter now controls whether
a remote disconnect erases the server Connection, so the only test that
needed to suppress that (the mp#348 getResults race test) just constructs
TestSetup with server_owns_connection=false instead of overriding a
callback afterward.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>