[Task]: Extract SSE logic from JSON-RPC client transport to share with REST implementation

See also:
- https://github.com/a2aproject/a2a-js/pull/169#discussion_r2517661823
- https://github.com/a2aproject/a2a-js/pull/169#discussion_r2517667781
- https://github.com/a2aproject/a2a-js/pull/169#discussion_r2517701417