Obsolete property in send email sample

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### This issue is for a: (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [ ] feature request
- [x] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

`poller.getOperationState().isStarted` was removed in a version of azure/communication-email as of the official Microsoft documentation points in favor of `poller.getOperationState().status !== "running"`. Reporting this here as the provided sample code still references this older property and may cause issues for developers who do not catch the notice in the documentation and rely solely on this sample.

Reference in MS docs: https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/send-email?tabs=windows%2Cconnection-string%2Csend-email-and-get-status-async%2Casync-client&pivots=programming-language-javascript

Issue in sample code file: `send-email/send-email.js` Line 27

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Open send-email/send-email.js at line 27 and compare the sample with the linked Microsoft documentation. Update the sample so it uses the current email poller status property, then verify the obsolete isStarted reference is gone and the sample still reflects the documented flow.

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