processDmCallStartEvent Path A now skips offline local members (matching
Path B's pre-existing per-member check) and pushes undeliverable when no
member could be rung, instead of creating a stranded FederatedCallEntry.
TDD — two new tests cover zero-online and mixed-online cases.
processDmCallStartEvent Path B no longer silently accepts when no local
participant is reachable. Pushes {messageId, reason: 'no_recipient'} to
the undeliverable ack bucket so the caller can surface fast-fail.
TDD — test asserts undeliverable push + no FederatedCallEntry.
Jannis Braun