[summarize-checks] Multiple NSTM comments race condition

https://github.com/Azure/azure-rest-api-specs/pull/36681

Looks like a race condition, where two instances of summarize-checks ran at exactly the same time, saw no existing comment, and added a new one.

https://github.com/Azure/azure-rest-api-specs/actions/runs/16980229351/job/48138479097#step:4:35

https://github.com/Azure/azure-rest-api-specs/actions/runs/16980229254/job/48138478854#step:4:35

Potential fix:

```js
const existingComments = getExistingComments(prNumber);
if (existingComments.contains(NSTM)) {
editComment();
}
else {
addNewComment();

// new code
const afterComments = getExistingComments(prNumber);
if (afterComments.containsMultiple(NSTM)) {
deleteCommentWeAdded();
updateExistingComment();
}
}
```