Can a Scrum Master also take on tasks such as writing test scripts or developing reports, especially now that GenAI tools make it easier for anyone to contribute? Traditionally, how was it viewed when Scrum Masters contributed in this way, and with GenAI potentially reducing team sizes, how might agile experts view this kind of involvement from Scrum Masters now?

As long as it doesn’t distract from their core role removing impediments and supporting continuous improvement.

My recommendation is to keep the Scrum Masters focused on their primary role of driving effectiveness and continuous improvements.  If you are considering stretching the scrum master goals due to limitations on the size of the team, the most important consideration is the experience and knowledge of the Scrum master.  

Does the scrum master has the experience to take on key activities like writing test scrips or developing reports?  Even using the help of AI tools requires a good level of understanding to ensure the right outcomes or results were generated.  If there are some tasks that can be delegated to help alleviate the team's size limitations as long as the expectations are clear and there is an initial level of verification from an expert until the skills and knowledge is fully acquired by the scrum master.