feat: improve thread safety and resource management in WebMVC streamable transport

- Add ReentrantLock synchronization to prevent race conditions in concurrent sseBuilder usage
- Add volatile closed flag to track session state and prevent operations on closed sessions
- Move SSE onComplete callback to properly close listening stream when connection completes
- Refactor closeGracefully to delegate to close() method for consistent cleanup
- Update class documentation to reflect thread-safety guarantees

This ensures safe concurrent access to SSE builder and proper cleanup of streaming resources.

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>