feat: add closeSseStream API for POST SSE polling (SEP-1699)

- Remove priming from GET streams (only POST streams per spec)
- Replace closeStandaloneSseStream() with closeSseStream(requestId)
- Add validation that retryInterval requires eventStore
- Add tests for POST SSE priming events and stream closure
- Restore original resumability tests without priming expectations

Author: felixweinberger