E2e s13 s16

[AYAHASSAN287]

Summary

Adds two scenario tests for the send() path:

• S13 — relay hard failure without fallback (passing)
• S16 — no delivery peers at T0, lightpush peer appears later (xfail, blocked by #3887)

Changes

test_send_errors_and_concurrency.py

Adds TestS13RelayHardFailureWithoutFallback.

A relay peer is connected so the publish gets past NO_PEERS_TO_RELAY and reaches the relay processor, where an oversized payload makes the publish fail. No lightpush fallback is configured. Verifies send() returns Ok(RequestId) followed by an Error event.

test_send_lightpush_and_edge.py

Adds TestS16LightpushPeerAppearsLater.

The edge sender starts with no reachable delivery peer; a lightpush service is meant to appear during the retry window so a later retry delivers the message. Verifies send() returns Ok(RequestId) followed eventually by Propagated.

S16 is marked xfail — see #3887 .

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