test(automations/templating): lock render, filters, environment, context

render.py (4): variable substitution, StrictUndefined raises on missing
keys, evaluate_predicate coerces to bool, render_value walks dicts/lists
and renders string leaves.

filters.py (4): slugify produces URL-safe output, date formats datetime
with strftime, date(None) → "" so templates can write
{{ inputs.last_fired_at | date }} on first run, date(str) passes through.

environment.py (4): the sandbox boundary — disallowed Jinja built-ins
(e.g. pprint) raise, and the finalize hook coerces non-string outputs
to predictable wire shapes (datetime → ISO, None → "", dict → JSON).

context.py (1): build_run_context exposes {run, inputs, steps} with the
exact shape every plan template body relies on.

13 tests total, all pure unit.

surfsense_backend/tests/unit/automations/templating/test_environment.py

@@ -0,0 +1,51 @@
+"""Lock the sandbox boundary: disallowed filters/tests reject, finalize coerces non-strings.
+
+These behaviors live in ``environment.py`` but are observed through the
+public ``render_template`` surface — the same surface every step uses.
+"""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+import pytest
+from jinja2.exceptions import TemplateError
+
+from app.automations.templating.render import render_template
+
+pytestmark = pytest.mark.unit
+
+
+def test_environment_rejects_filters_not_in_the_allowlist() -> None:
+    """A template that pipes through a Jinja built-in **not** in the
+    allowlist (e.g. ``pprint``) must fail rather than rendering. Locks
+    the sandbox surface against accidental re-introduction of removed
+    filters."""
+    with pytest.raises(TemplateError):
+        render_template("{{ value | pprint }}", {"value": {"k": 1}})
+
+
+def test_environment_finalizes_datetime_output_to_iso_string() -> None:
+    """A datetime that lands directly at an output site is stringified
+    via ``isoformat()`` rather than producing ``str(datetime)`` (which
+    has a space separator). Locks the wire shape templates produce
+    when emitting ``inputs.fired_at`` and other datetime values."""
+    dt = datetime(2026, 5, 28, 14, 30, tzinfo=UTC)
+
+    assert render_template("{{ moment }}", {"moment": dt}) == "2026-05-28T14:30:00+00:00"
+
+
+def test_environment_finalizes_none_output_to_empty_string() -> None:
+    """A ``None`` at an output site becomes the empty string. Lets
+    templates write ``{{ inputs.last_fired_at }}`` unconditionally on
+    the first run without exploding on the null."""
+    assert render_template("{{ missing }}", {"missing": None}) == ""
+
+
+def test_environment_finalizes_dict_output_to_json() -> None:
+    """A dict at an output site is JSON-serialized. Same for lists.
+    Locks the wire shape so users embedding structured values into
+    prompts get deterministic, parseable output."""
+    rendered = render_template("{{ payload }}", {"payload": {"a": 1, "b": [2, 3]}})
+
+    assert rendered == '{"a": 1, "b": [2, 3]}'