fix(cli): finishedAt type in SyncStatus (#4013)

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`finishedAt` can be undefined before the first run.
https://nangohq.slack.com/archives/C069QNE69DX/p1745529633956029

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<!-- Summary by @propel-code-bot -->

---

This PR updates the `finishedAt` field type in the SyncStatus interface
from `string` to `string | undefined` to properly handle the case where
the field is undefined before the first sync run.

*This summary was automatically generated by @propel-code-bot*

Author: kaposke

packages/node-client/lib/types.ts

@@ -193,7 +193,7 @@ export interface Integration {
 export interface SyncStatus {
     id: string;
     type: SyncType;
-    finishedAt: string;
+    finishedAt: string | undefined;
     nextScheduledSyncAt: string;
     name: string;
     status: 'RUNNING' | 'SUCCESS' | 'ERROR' | 'PAUSED' | 'STOPPED';