Fix delete_image: use sha256 image ID instead of name+tag

DSM SYNO.Docker.Image/delete returns error 114 when called with name+tag.
The API expects the sha256 hash from the image list (field "id") as the
"id" parameter.

- Look up the sha256 hash from SYNO.Docker.Image/list (already fetched
  for the in-use check), then pass params={"id": img_hash}
- Guard against missing hash with a clear error message
- Updated tests to assert id param is sent, name/tag are absent

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_modules/test_images.py

@@ -225,6 +225,13 @@ async def test_delete_image_confirmed():
     assert "redis:7" in result
     assert "freed" in result
 
+    # Delete must use the sha256 id, not name+tag
+    delete_call = next(c for c in client.request.call_args_list if c.args[1] == "delete")
+    params = delete_call.kwargs.get("params") or {}
+    assert params.get("id") == "sha256:cccc"
+    assert "name" not in params
+    assert "tag" not in params
+
 
 @pytest.mark.asyncio
 async def test_delete_image_not_found():
@@ -330,11 +337,12 @@ async def test_delete_image_registry_prefixed_name():
     assert "Deleted" in result
     assert "open-webui" in result
 
-    # Verify delete was called with correct split params (not "ghcr" as name)
+    # Verify delete was called with the sha256 ID, not name+tag
     delete_call = next(c for c in client.request.call_args_list if c.args[1] == "delete")
     params = delete_call.kwargs.get("params") or {}
-    assert params.get("name") == "ghcr.io/open-webui/open-webui"
-    assert params.get("tag") == "v0.8.10"
+    assert params.get("id") == "sha256:dddd"
+    assert "name" not in params
+    assert "tag" not in params
 
 
 @pytest.mark.asyncio