Obsolete property in send email sample

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### This issue is for a: (mark with an `x`)
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- [ ] feature request
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`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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打开第 27 行的 send-email/send-email.js，并将示例与链接的 Microsoft 文档进行比较。更新示例，使其使用当前的电子邮件轮询器状态属性，然后验证过时的 isStarted 引用已被移除，并且示例仍然反映文档中所述的流程。

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