fix(plugin): normalize event session ids

Handle OpenCode session events that carry the session ID under properties.info.id or properties.info.sessionID so background tasks and continuation hooks do not miss idle/error/delete events.

Add regression coverage for nested session.idle events completing background tasks and waking continuation hooks.

src/hooks/session-notification-event-properties.ts

@@ -23,6 +23,15 @@ export function getSessionID(properties: EventProperties): string | undefined {
   const infoSessionId = info?.sessionId
   if (typeof infoSessionId === "string" && infoSessionId.length > 0) return infoSessionId
 
+  const part = properties?.part
+  if (isRecord(part)) {
+    const partSessionID = part.sessionID
+    if (typeof partSessionID === "string" && partSessionID.length > 0) return partSessionID
+
+    const partSessionId = part.sessionId
+    if (typeof partSessionId === "string" && partSessionId.length > 0) return partSessionId
+  }
+
   return undefined
 }