refactor(jobs): address code review feedback on fp.job.step (Task 1.6)

- I1: Replace 'Task 1.6' markers in stub method comments and
  NotImplementedError messages with forward-looking phrasing.
  Task 1.6 is what just shipped (the field expansion); the action
  stubs are deferred to an unspecified future task. Stale markers
  would have confused future readers/operators.
- I2: Add test_cost_total_recomputes_when_rate_changes — insurance
  test that verifies @api.depends('cost_per_hour') triggers through
  the related-from-work_centre chain. Catches future Odoo upgrades
  that break related-depends.

Manifest 19.0.8.5.0 → 19.0.8.5.1.

Part of: native job model migration (spec 2026-04-25)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating/tests/test_fp_job_step_state_machine.py

@@ -103,3 +103,20 @@ class TestFpJobStepStateMachine(TransactionCase):
         step.duration_actual = 30.0
         # Recompute happens on read after a write to a depends field
         self.assertEqual(step.cost_total, 30.0)
+
+    def test_cost_total_recomputes_when_rate_changes(self):
+        # Insurance test: verify @api.depends('cost_per_hour') triggers
+        # through the related-from-work_centre chain. If a future Odoo
+        # upgrade breaks related-depends, this test catches it.
+        wc = self.env['fp.work.centre'].create({
+            'name': 'WC4', 'code': 'WC4', 'kind': 'wet_line',
+            'cost_per_hour': 60.0,
+        })
+        step = self._make_step(work_centre_id=wc.id)
+        step.duration_actual = 30.0
+        self.assertEqual(step.cost_total, 30.0)
+        # Change the rate; cost_total should recompute.
+        wc.cost_per_hour = 120.0
+        # Force recompute via invalidate (Odoo recomputes on next read).
+        step.invalidate_recordset(['cost_total'])
+        self.assertEqual(step.cost_total, 60.0)