test: comprehensive shadow channel tests (15 cases)

Rewrite test suite for SSE channel replacement fix:
- Shadow creation: standalone GET returns 200, multiple GETs coexist
- Dead primary: replacement, notification delivery, repeated cycles
- Notification routing: primary receives, shadow does not
- Resume paths: completed request-wise, common alive/dead
- Real scenarios: Cursor leapfrog, VS Code reconnect
- Edge cases: invalid session, missing header, shadow cleanup

Fix Accept header bug (was missing text/event-stream for
notifications/initialized POST, causing 406 rejection).

Author: its-mash