fix: align autostart tests with persisted next_start_at (#26037)

`TestExecutorAutostartOK` and its sibling positive autostart tests
compute the autobuild tick from
`sched.Next(workspace.LatestBuild.CreatedAt)`, but the server persists
`next_start_at` from the build's completion time. When build creation
and completion straddle the schedule's next fire time, the persisted
value advances past the test's tick, the executor's eligibility query
(`next_start_at <= tick`) drops the workspace, and the test fails with
an empty transitions map. This surfaced in flaky test runs.

Add `coderdtest.NextAutostartTick(t, workspace)` which returns
`*workspace.NextStartAt`, and use it across the affected positive
autostart paths in `coderd/autobuild`, `coderd`, and
`enterprise/coderd`.

Generated with assistance from Coder Agents.

coderd/workspaces_test.go

@@ -6212,8 +6212,8 @@ func TestWorkspaceBuildsEnqueuedMetric(t *testing.T) {
 	p, err := coderdtest.GetProvisionerForTags(db, time.Now(), workspace.OrganizationID, map[string]string{})
 	require.NoError(t, err)
 
+	tickTime := coderdtest.NextAutostartTick(t, workspace)
 	go func() {
-		tickTime := sched.Next(workspace.LatestBuild.CreatedAt)
 		coderdtest.UpdateProvisionerLastSeenAt(t, db, p.ID, tickTime)
 		tickCh <- tickTime
 		close(tickCh)