feat: add backend logic for determining tasks tab visibility (#18401)

This PR implements the backend logic for determining if the Tasks tab
should be visible in the web UI as described in [the
RFC](https://www.notion.so/coderhq/Coder-Tasks-207d579be5928053ab68c8d9a4b59eaa?source=copy_link#210d579be5928013ab5acbe69a2f548b).

The frontend component will be added in a follow-up PR once the entire
Tasks backend is implemented so as not to break the dogfood environment
until then.

coderd/database/queries/templateversions.sql

@@ -226,3 +226,7 @@ FROM
 WHERE
 	template_versions.id IN (archived_versions.id)
 RETURNING template_versions.id;
+
+-- name: HasTemplateVersionsWithAITask :one
+-- Determines if the template versions table has any rows with has_ai_task = TRUE.
+SELECT EXISTS (SELECT 1 FROM template_versions WHERE has_ai_task = TRUE);