Triage transport unit tests around the removed router internals

The per-session message router and its stream fan-out are gone, so the
tests pinning that mechanism are rewritten against the behaviour they
guarded (priming-store failure returns 500 with no leaked state; standalone
stream teardown via close_standalone_sse_stream logs no error; the manager
evicts sessions whose task exits or crashes; stateless requests leave no
channels behind) or deleted where the guarded race is now unrepresentable
(#1764 router head-of-line blocking, the standalone writer between-dequeues
window). The close_sse_stream protocol-version gating tests move to the
renamed metadata builder.

Author: maxisbey