fix(backend): idempotent spawnFor + identity-checked exit + unified SSE re-emit shape

- spawnFor now tears down any existing runtime for the same session id before
  the cap check, so resume/respawn neither leaks the old child nor falsely hits
  maxPiProcesses
- exit handler is identity-checked (captures rt) so a stale child's late exit
  cannot evict a newer runtime
- SSE pending re-emit now sends the full ClientEvent shape
  { type: "ui_request", ui_request } to match hub.publish live events
- tests: same-id respawn replaces runtime (count 1); old child exit does not
  evict new runtime; sse-hub re-emit asserts unified shape

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/sse/sse-hub.ts

@@ -4,7 +4,9 @@ export class SseHub {
   subscribe(sessionId: string, send: Send, pending?: object | null): () => void {
     if (!this.subs.has(sessionId)) this.subs.set(sessionId, new Set());
     this.subs.get(sessionId)!.add(send);
-    if (pending) send("ui_request", { ui_request: pending });
+    // Match the live-event shape (hub.publish sends the full ClientEvent):
+    // both carry { type, ui_request } so re-emit and live widgets are identical.
+    if (pending) send("ui_request", { type: "ui_request", ui_request: pending });
     return () => this.subs.get(sessionId)?.delete(send);
   }
   publish(sessionId: string, event: string, data: object): void {