test(triggers-api): pin UTC datetime serialization at the boundary

Five cases lock down the rule that both ``_iso`` and the
``TriggerJobRead`` Pydantic serializer emit an explicit UTC offset
regardless of whether the source datetime is tz-aware or naive (the
SQLite read path).

src/backend/tests/unit/services/triggers/test_datetime_serialization.py

@@ -0,0 +1,64 @@
+"""Tests for UTC normalization at the triggers HTTP boundary.
+
+SQLite drops the tzinfo of ``DateTime(timezone=True)`` columns on
+read, so naive datetimes can leak into the API layer. Both the
+manual aggregator serializer (``_iso``) and the SQLModel
+``TriggerJobRead`` must coerce them to UTC-aware ISO strings so the
+browser does not interpret them in its local timezone.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime, timezone
+from uuid import uuid4
+
+from langflow.api.v1.triggers import _iso
+from langflow.services.database.models.jobs.model import JobStatus
+from langflow.services.database.models.triggers.model import TriggerJobRead
+
+
+def test_iso_emits_utc_offset_for_aware_datetime():
+    value = datetime(2026, 5, 22, 0, 44, 0, tzinfo=timezone.utc)
+    assert _iso(value) == "2026-05-22T00:44:00+00:00"
+
+
+def test_iso_treats_naive_datetime_as_utc():
+    value = datetime(2026, 5, 22, 0, 44, 0)  # noqa: DTZ001 — simulates SQLite read
+    assert _iso(value) == "2026-05-22T00:44:00+00:00"
+
+
+def test_iso_returns_none_for_missing_value():
+    assert _iso(None) is None
+
+
+def _job_read(**overrides) -> TriggerJobRead:
+    base = {
+        "id": uuid4(),
+        "flow_id": uuid4(),
+        "component_id": "CronTrigger-x",
+        "status": JobStatus.COMPLETED,
+        "scheduled_at": datetime(2026, 5, 22, 0, 44, 0),  # noqa: DTZ001
+        "started_at": None,
+        "finished_at": None,
+        "attempt": 1,
+        "max_attempts": 3,
+        "error": None,
+        "run_job_id": None,
+        "created_at": datetime(2026, 5, 22, 0, 43, 0),  # noqa: DTZ001
+    }
+    base.update(overrides)
+    return TriggerJobRead(**base)
+
+
+def test_trigger_job_read_serializes_naive_datetime_as_utc():
+    dumped = _job_read().model_dump(mode="json")
+    assert dumped["scheduled_at"] == "2026-05-22T00:44:00+00:00"
+    assert dumped["created_at"] == "2026-05-22T00:43:00+00:00"
+    assert dumped["started_at"] is None
+    assert dumped["finished_at"] is None
+
+
+def test_trigger_job_read_preserves_aware_datetime():
+    aware = datetime(2026, 5, 22, 3, 44, 0, tzinfo=timezone.utc)
+    dumped = _job_read(scheduled_at=aware).model_dump(mode="json")
+    assert dumped["scheduled_at"] == "2026-05-22T03:44:00+00:00"