Reproduces the two-step handshake the client performs — POST thread/withmembers
to create the thread, then GET thread/{id} to open it — on a conversation that
has never carried a message, which is the case that was 404ing.
The request is sent as a form body rather than a query string, matching the
wire ("ids=9741717&messageCount=50"). That detail is load-bearing for routing,
not just fidelity: POST thread/{chatThreadId} also matches the literal path
thread/withmembers, and the content type is what separates them. Posting with
no body makes the two indistinguishable and the request fails as an ambiguous
match across CreateGroup, WithMembersForm and SendMessageForm.
Alexa