text: Title: Overwriting dirname causes test suite not to fail if snapshots unused
  Body: **Describe the bug**
  
  Within Home Assistant we have a SnapshotExtension to overwrite the default `__snapshots__` dir name with `snapshots`. But today I actually noticed that this had the side effect that it doesn't fail the test suite if there are unused snapshots in the snapshots folder.
  
  ```py
  class HomeAssistantSnapshotExtension(AmberSnapshotExtension):
      """Home Assistant extension for Syrupy."""
  
      VERSION = "1"
      """Current version of serialization format.
  
      Need to be bumped when we change the HomeAssistantSnapshotSerializer.
      """
  
      serializer_class: type[AmberDataSerializer] = HomeAssistantSnapshotSerializer
  
      @classmethod
      def dirname(cls, *, test_location: PyTestLocation) -> str:
          """Return the directory for the snapshot files.
  
          Syrupy, by default, uses the `__snapshosts__` directory in the same
          folder as the test file. For Home Assistant, this is changed to just
          `snapshots` in the same folder as the test file, to match our `fixtures`
          folder structure.
          """
          test_dir = Path(test_location.filepath).parent
          return str(test_dir.joinpath("snapshots"))
  
  ```
  
  **To reproduce**
  Add the SnapshotExtension. Have it generate a snapshot. Add a fake entry to the snapshot. Run the test suite.
  
  **Expected behavior**
  
  I'd expect it to fail the test suite because there are entries that are not found. I can imagine that we have quite a basic way of overwriting the directory and can imagine people with way more interesting setups for the snapshots which would probably break with the expected behaviour. So I would not be surprised if the failure is turned off when the dirname is overwritten, but I'd then at least expect to read something in a docstring or in the docs.
  
  **Screenshots**
  
  <!-- If applicable, add screenshots to help explain your problem. -->
  
  **Environment (please complete the following information):**
  
  - OS: mac OS
  - Syrupy Version: 4.6.1
  - Python Version: 3.12
  
  **Additional context**
  
  <!-- Add any other context about the problem here. -->
  