[Feat] BLS SSE endpoint – session stream as NDJSON from session state progress schema

[pregnor]

Phase 3, State API, SSE, and A2A middleware

Implement GET /agent/sessions/{session_id}/stream (or equivalent) that streams events/state deltas for that session as NDJSON, with each line conforming to the same A2A message schema defined for the session state progress payload.

• Subscribe to the session’s event stream from the store ([Feat] BLS In-memory session state and event store #448). Each item pushed to the subscriber is emitted as one NDJSON line.
• Content of each line must conform to the JSON schema from [Feat] BLS Define JSON schema for session state progress payload #446 so frontend and backend share one contract for both A2A and SSE.
• At least one SSE endpoint per session; connection lifecycle and auth are out of scope unless the epic specifies them.