nsupdate: add unit tests (#11677)
* nsupdate: add unit tests
* fix var name to regain sanity
* remove unneeded typing from test file
* formatting
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(cherry picked from commit ef700b116a)
Co-authored-by: Alexei Znamensky <103110+russoz@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
