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/runtime-fallback/message-update-handler.ts

@@ -8,6 +8,7 @@ import { getFallbackModelsForSession } from "./fallback-models"
 import { resolveFallbackBootstrapModel } from "./fallback-bootstrap-model"
 import { dispatchFallbackRetry } from "./fallback-retry-dispatcher"
 import { hasVisibleAssistantResponse } from "./visible-assistant-response"
+import { resolveMessageEventSessionID } from "../../shared/event-session-id"
 
 export { hasVisibleAssistantResponse } from "./visible-assistant-response"
 
@@ -17,7 +18,7 @@ export function createMessageUpdateHandler(deps: HookDeps, helpers: AutoRetryHel
 
   return async (props: Record<string, unknown> | undefined) => {
     const info = props?.info as Record<string, unknown> | undefined
-    const sessionID = info?.sessionID as string | undefined
+    const sessionID = resolveMessageEventSessionID(props)
     const timeoutEnabled = config.timeout_seconds > 0
     const eventParts = props?.parts as Array<{ type?: string; text?: string }> | undefined
     const infoParts = info?.parts as Array<{ type?: string; text?: string }> | undefined