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/shared/event-session-id.ts

@@ -0,0 +1,23 @@
+import { isRecord } from "./record-type-guard"
+
+function getStringField(record: Record<string, unknown> | undefined, key: string): string | undefined {
+  const value = record?.[key]
+  return typeof value === "string" && value.length > 0 ? value : undefined
+}
+
+export function resolveSessionEventID(properties: unknown): string | undefined {
+  const props = isRecord(properties) ? properties : undefined
+  const info = isRecord(props?.info) ? props.info : undefined
+  return getStringField(props, "sessionID")
+    ?? getStringField(info, "sessionID")
+    ?? getStringField(info, "id")
+}
+
+export function resolveMessageEventSessionID(properties: unknown): string | undefined {
+  const props = isRecord(properties) ? properties : undefined
+  const info = isRecord(props?.info) ? props.info : undefined
+  const part = isRecord(props?.part) ? props.part : undefined
+  return getStringField(props, "sessionID")
+    ?? getStringField(info, "sessionID")
+    ?? getStringField(part, "sessionID")
+}