Add validation to the rules created via assistant manually

Users can modify rule text offered by the assistant. If a selector in the user rule is invalid, the assistant should notify the user about it.
It would mitigate cases described in the #1580

The issue names no files, tests, or entry points. Trace the assistant flow that creates and lets users modify rules, then identify where selectors are handled; done means an invalid selector in a manually modified rule is detected and the user is notified, covering the cases referenced by #1580.

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