SSE events are missing their terminating blank line

The Server-Sent Events spec only dispatches an event to the client's EventSource.onmessage once a blank line terminates it. If that blank line is missing, a strict SSE parser keeps buffering fields, waiting for the terminator, and only dispatches once something else eventually supplies one -- so every notification arrives one event late, and if a user's session ends after their last notification, that final notification may never be dispatched to the browser at all. For a notifications feature, the last thing that happened being the one thing the user never finds out about is close to the worst possible failure mode.

format_sse builds each SSE chunk by joining its lines with a single newline and appending one more newline at the end, producing exactly one trailing newline -- one short of the blank line (two consecutive newlines) the SSE spec requires to terminate an event.

1. cd fastapi/sse_notifications
2. python3 -c "from app.streaming import format_sse; c = format_sse('1-0', 'message', '{}'); print(repr(c)); print(c.endswith(chr(10)*2))"
3. Observe: prints False -- the chunk ends in one newline, not the required blank-line terminator.

This is the kind of bug a substring-based test (asserting "id: X" in chunk) will never catch, because the field content is completely correct -- only the terminator is missing. It takes either a strict byte-for-byte check of the chunk's ending, or an actual SSE-parsing client, to notice anything is wrong. HEARTBEAT_COMMENT right above this function gets the terminator right, which makes the asymmetry easy to miss on a quick read.

Compare format_sse's return value against HEARTBEAT_COMMENT's literal ending. "\n".join(lines) only puts a newline between elements -- think about what belongs at the end of lines for the join to naturally produce a trailing blank line.