Rename build_tag to build in validate_input() #1091

ActorClient.validate_input() (and its async twin) takes the build as build_tag= (_resource_clients/actor.py:566). Everywhere else the client calls the same thing build= (start(), call()), and the JS client's validateInput() also uses build.

Rename the parameter to build. To keep this non-breaking until the next major, accept build_tag as a deprecated alias that emits a DeprecationWarning, then drop it in the major.

✍️ Drafted by Claude Code