Technically any phone can "cast video to a Roku Smart TV" - however, it all comes down to the software, be it firmware or apps, that handle the connection, and how.
Roku supports Miracast and AirPlay2 (4k Models and certain Roku TVs) for screen mirroring, so look for phones that support that.
If you prefer Android-based, find one that specifically mentions Miracast support - Most Samsung models have built-in Miracast support using their SmartView module - however some models lack SmartView entirely, so check before you buy.
