[core] Fix area/device hash collision validation not running (#15259)

tests/unit_tests/core/test_config.py

@@ -248,6 +248,24 @@ def test_area_id_hash_collision(
     )
 
 
+def test_area_singular_hash_collision(
+    yaml_file: Callable[[str], str], capsys: pytest.CaptureFixture[str]
+) -> None:
+    """Test that area hash collisions between singular area: and areas: list are detected."""
+    result = load_config_from_fixture(
+        yaml_file, "area_singular_hash_collision.yaml", FIXTURES_DIR
+    )
+    assert result is None
+
+    captured = capsys.readouterr()
+    assert (
+        "Area ID 'd6ka' with hash 3082558663 collides with existing area ID 'test_2258'"
+        in captured.out
+    )
+    # Error path should point to 'areas' (where the colliding entry is), not 'area'
+    assert "areas" in captured.out
+
+
 def test_device_duplicate_id(
     yaml_file: Callable[[str], str], capsys: pytest.CaptureFixture[str]
 ) -> None:

tests/unit_tests/fixtures/core/config/area_singular_hash_collision.yaml

@@ -0,0 +1,10 @@
+esphome:
+  name: test
+  area:
+    id: test_2258
+    name: "Area 1"
+  areas:
+    - id: d6ka
+      name: "Area 2"
+
+host: