Server-side SSE per-event spans are missing #1216

When a Server Agent sends SSE streaming events back to the Client, there are no per-event spans on the Server side. The Server span ends as soon as the stream publisher is created ("Stream publisher created"), making the actual event emission process a complete black box.
Current Behavior

Client Agent Server Agent
───────── ─────────
[CLIENT Span: sendMessage] ──HTTP──→ [SERVER Span: onMessageSendStream]
│ │
│ Span ends after stream init │ Span ends after publisher returns
▼ ▼
[CLIENT Span: event-1] ❌ No Span
[CLIENT Span: event-2] ❌ No Span
[CLIENT Span: event-3] ❌ No Span
│ │
▼ ▼
Link to initial Event sending completely
CLIENT Span (non-parent-child) untraceable

The Server span closes before any SSE events are actually sent.
The Client side has per-event spans (linked via addLink), but the Server side has nothing.
Client and Server event spans cannot be correlated one-to-one.

Expected Behavior

Each SSE event sent by the Server should have an INTERNAL span as a child of the Server span (via context restoration), with attributes such as streaming.event.kind (status / artifact / completed / error). This would close the end-to-end trace loop.
Impact

Server-side event emission is a black box — no visibility into which events were sent, when, or how long each took.
End-to-end trace tree is broken: the Server span has no children, and Client event spans are only linked (not parent-child), so they don't appear in the trace tree view.
Cannot correlate Client-side event spans with Server-side event sending.

cf Issue #1132