Concurrent activation requests for the same version can race: another request may materialize and activate the task first, and Evergreen returns 400 for activation requests with nothing left to activate.

Rather than pre-checking activation state (which narrows the TOCTOU window instead of closing it), call `activate_version_tasks` unconditionally and handle the 400 after the fact: re-look up the task and only swallow the error once a fresh lookup confirms it is activated. Non-400 errors and genuinely-unactivated tasks propagate.

Same underlying issue as SERVER-117496.