sendFederatedCallStart now treats a 200-with-undeliverable-messageId as
a peer-failure instead of unconditional success. Feeds the existing
failures[] array and terminal-determination machinery from #16.
New sendFederatedCallStartForTest export mirrors the existing
handleDm*ForTest pattern. TDD — three tests cover single-peer terminal
no_recipient, group-DM mixed delivered+undeliverable non-terminal, and
the happy-path (empty undeliverable → no event).
Also hardens sendCallRelay's response parse: validates undeliverable
is an Array and entries are well-shaped, logs protocol drift at warn/debug
rather than silently falling back to old-peer semantics.
Jannis Braun
26a4925032