Move the /tasks/* sub-route out of _dispatch into _dispatch_task_route
(query passed explicitly; the files/tree/thread branches use it), move
the push-subscription DELETE branch out of _dispatch_project into
_project_push_subscription_delete, and turn the goal-validation block
of _create_task into the pure module function
_validate_task_goal_options (returns error string instead of sending
400, caller sends it unchanged). Statements are moved verbatim except
indentation and the goal send+return -> return-error transform; no
behaviour change. server.py module split intentionally not done: it
remains a larger, higher-risk refactor.
Add test_longfunc_split covering the pure goal validator's success
paths and all eight error strings.
