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/dbmem/dbmem.go

@@ -8495,6 +8495,19 @@ func (q *FakeQuerier) GetWorkspacesEligibleForTransition(ctx context.Context, no
 	return workspaces, nil
 }
 
+func (q *FakeQuerier) HasTemplateVersionsWithAITask(_ context.Context) (bool, error) {
+	q.mutex.RLock()
+	defer q.mutex.RUnlock()
+
+	for _, templateVersion := range q.templateVersions {
+		if templateVersion.HasAITask {
+			return true, nil
+		}
+	}
+
+	return false, nil
+}
+
 func (q *FakeQuerier) InsertAPIKey(_ context.Context, arg database.InsertAPIKeyParams) (database.APIKey, error) {
 	if err := validateDatabaseType(arg); err != nil {
 		return database.APIKey{}, err