Web Shell session-created callback

Goal

Let an embedding host run asynchronous work with a newly created session ID before Web Shell continues session initialization or submits the first prompt.

Design

Add an optional onSessionCreated(sessionId) prop to WebShell. The lazy session-creation path awaits it immediately after createSession succeeds and before attaching the session. A rejected callback aborts the first prompt and uses the existing unattached-session cleanup path. The callback times out after 30 seconds so a host cannot permanently block setup or leak the unattached session.

Existing sessions loaded through the sessionId prop or session picker do not invoke the callback because Web Shell did not create them.