Extends PR #15171 to also cover the server-side cancellation path (aiohttp
shutdown, request-level timeout) — previously only ConnectionResetError
triggered the incomplete-snapshot write, so cancellations left the store
stuck at the in_progress snapshot written on response.created.
Factors the incomplete-snapshot build into a _persist_incomplete_if_needed()
helper called from both the ConnectionResetError and CancelledError
branches; the CancelledError handler re-raises so cooperative cancellation
semantics are preserved.
Adds two regression tests that drive _write_sse_responses directly (the
TestClient disconnect path races the server handler, which makes the
end-to-end assertion flaky).
36d68bcb82