For other critical extensions, on top of the blocking-CI selenium tests which stop us from landing changes that break our tests, we also have daily jobs that run on the Beta Cluster with the full gamut of production code, e.g. for AdvancedSearch or VisualEditor. It would be a good step for peace of mind to do the same for WikiLambda / Wikifunctions.
Hi and thank you for your interest! Please check thoroughly https://www.mediawiki.org/wiki/New_Developers (and all of its communication section!). The page covers how to get started, assigning tasks, task status, how to find a codebase, how to create patches, where to ask general development questions and where to get help with setup problems, and how to ask good questions. Thanks a lot! :)