Use active worktree's task sources (#25784)

Follow-up of https://github.com/zed-industries/zed/pull/25605

Previous PR made global tasks with `ZED_WORKTREE_ROOT` available for
"nothing open" scenario, this PR also gets all related worktree task
templates, using the centralized `TextContexts`' active worktree
detection.

Release Notes:

- N/A