fix(fusion_tasks): make datetime inverses use the same tz resolver as compute

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_tasks/tests/__init__.py

@@ -0,0 +1,2 @@
+# -*- coding: utf-8 -*-
+from . import test_task_tz

fusion_tasks/tests/test_task_tz.py

@@ -0,0 +1,39 @@
+# -*- coding: utf-8 -*-
+from datetime import date
+from odoo.tests.common import TransactionCase, tagged
+
+
+@tagged('post_install', '-at_install')
+class TestTaskTz(TransactionCase):
+
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+        # _compute_datetimes resolves company resource-calendar tz FIRST, then user tz.
+        # Set BOTH to Toronto so the UTC assertion and the round-trip are deterministic.
+        cls.env.user.tz = 'America/Toronto'
+        cal = cls.env.company.resource_calendar_id
+        if cal:
+            cal.tz = 'America/Toronto'
+        # technician_id is required (domain x_fc_is_field_staff=True) -> make a field tech.
+        cls.tech = cls.env['res.users'].create({
+            'name': 'TZ Test Tech',
+            'login': 'tz_test_tech_svcbook',
+            'x_fc_is_field_staff': True,
+        })
+        cls.task = cls.env['fusion.technician.task'].create({
+            'technician_id': cls.tech.id,
+            'scheduled_date': date(2026, 6, 3),
+            'time_start': 9.0,
+            'time_end': 10.0,
+        })
+
+    def test_local_to_utc_compute(self):
+        # 9:00 local Toronto (EDT, -4) -> 13:00 UTC stored
+        self.assertEqual(self.task.datetime_start.hour, 13)
+
+    def test_inverse_round_trips_with_same_tz(self):
+        # writing datetime_start back recovers the same local time_start
+        self.task.datetime_start = self.task.datetime_start  # force inverse
+        self.task.flush_recordset(['datetime_start'])
+        self.assertAlmostEqual(self.task.time_start, 9.0, places=2)