M8-R14: preserve zero DSMR sample interval

tests/test_meter_source_services.py

@@ -68,6 +68,32 @@ def test_secret_sanitize_and_mask_merge_keep_old_value():
     assert merged["topic"] == "new/topic"
 
 
+def test_dsmr_zero_interval_validates_sanitizes_and_merges_unchanged():
+    config = validate_source_config("dsmr_mqtt", {"sample_interval_s": 0})
+
+    assert config["sample_interval_s"] == 0
+    assert sanitize_source_config("dsmr_mqtt", config)["sample_interval_s"] == 0
+    assert merge_source_config("dsmr_mqtt", config, {"topic": "new/topic"})["sample_interval_s"] == 0
+
+
+@pytest.mark.parametrize(
+    ("kind", "config"),
+    [
+        ("dsmr_mqtt", {"sample_interval_s": -1}),
+        ("dsmr_mqtt", {"sample_interval_s": False}),
+        ("dsmr_mqtt", {"broker_port": 0}),
+        ("dsmr_mqtt", {"broker_port": -1}),
+        ("dsmr_mqtt", {"broker_port": False}),
+        ("warmtelink_serial", {"path": "/dev/warmtelink", "baudrate": 0}),
+        ("warmtelink_serial", {"path": "/dev/warmtelink", "baudrate": -1}),
+        ("warmtelink_serial", {"path": "/dev/warmtelink", "baudrate": False}),
+    ],
+)
+def test_numeric_source_profile_constraints_still_reject_invalid_values(kind, config):
+    with pytest.raises(SourceProfileError):
+        validate_source_config(kind, config)
+
+
 @pytest.fixture()
 def session(tmp_path):
     engine = create_engine(f"sqlite:///{tmp_path / 'source_services.db'}")