closes https://github.com/unoplatform/uno-private/issues/1353

Bugfix

Fix multiple instance of dev-server can be started when re-using VS sinatnce with multiple solutions

What is the current behavior?

• Previous instances of EntryPoint leaks and also starts dev-server
• Concurrency issue could cause dev-server to be started more than once

What is the new behavior?

• Make sure to un-subscribe event to avoid duplicated work + safety check before stating the server
• All the logic to start the dev-server is gated

PR Checklist

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

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This is an automatic backport of pull request #20618 done by [Mergify](https://mergify.com).