M8-R11: make business timezone deterministic

tests/test_timezone.py

@@ -7,7 +7,7 @@ on any CI host timezone.
 from __future__ import annotations
 
 from datetime import UTC, date, datetime
-from zoneinfo import ZoneInfo
+from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
 
 import pytest
 
@@ -56,6 +56,38 @@ def test_local_tz_env_var_overrides(monkeypatch):
     monkeypatch.delenv("TZ")
 
 
+@pytest.mark.parametrize("value", [None, "", "   "])
+def test_local_tz_defaults_to_dst_aware_amsterdam(monkeypatch, value: str | None):
+    """Unset or blank TZ must use the deterministic Amsterdam business zone."""
+    if value is None:
+        monkeypatch.delenv("TZ", raising=False)
+    else:
+        monkeypatch.setenv("TZ", value)
+
+    tz = tz_mod.local_tz()
+    assert isinstance(tz, ZoneInfo)
+    assert tz.key == "Europe/Amsterdam"
+    winter = datetime(2026, 1, 15, 12, tzinfo=tz)
+    summer = datetime(2026, 7, 15, 12, tzinfo=tz)
+    assert winter.utcoffset().total_seconds() == 3600
+    assert summer.utcoffset().total_seconds() == 7200
+
+
+def test_local_tz_explicit_utc_override(monkeypatch):
+    """A non-empty TZ remains an operator-controlled override."""
+    monkeypatch.setenv("TZ", "UTC")
+    tz = tz_mod.local_tz()
+    assert isinstance(tz, ZoneInfo)
+    assert tz.key == "UTC"
+
+
+def test_local_tz_invalid_explicit_override_fails_loudly(monkeypatch):
+    """A non-empty invalid override must not silently become Amsterdam."""
+    monkeypatch.setenv("TZ", "Invalid/Timezone")
+    with pytest.raises(ZoneInfoNotFoundError):
+        tz_mod.local_tz()
+
+
 # ---------------------------------------------------------------------------
 # to_local() — conversion correctness
 # ---------------------------------------------------------------------------