Fix delete_image: use rpartition for name:tag splitting

partition(":") split at the first colon, so registry-prefixed images
like "ghcr.io/open-webui/open-webui:v0.8.10" produced name="ghcr"
instead of the correct repository name, causing DSM error 114.

rpartition(":") always splits at the last colon, correctly handling:
- plain images: "nginx:1.24" → name="nginx", tag="1.24"
- namespaced: "nouchka/sqlite3:latest" → correct split
- registry URLs: "ghcr.io/foo/bar:v1" → name="ghcr.io/foo/bar", tag="v1"
- no tag: "nginx" → name="nginx", tag="latest" (fallback)

Added regression test verifying correct params sent to DSM delete API.

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

tests/test_modules/test_images.py

@@ -291,6 +291,52 @@ async def test_delete_image_by_hash():
     assert "redis" in result
 
 
+@pytest.mark.asyncio
+async def test_delete_image_registry_prefixed_name():
+    """Registry-prefixed image names (e.g. ghcr.io/foo/bar:v1) must split at last ':'."""
+    from mcp_synology_container.modules.images import register_images
+
+    registry_images = {
+        "images": [
+            {
+                "id": "sha256:dddd",
+                "repository": "ghcr.io/open-webui/open-webui",
+                "tags": ["v0.8.10"],
+                "size": 100 * 1024 * 1024,
+                "created": 1700000000,
+                "upgradable": False,
+            }
+        ]
+    }
+
+    client = AsyncMock()
+
+    async def mock_request(api, method, **kwargs):
+        if api == "SYNO.Docker.Image" and method == "list":
+            return registry_images
+        if api == "SYNO.Docker.Container":
+            return {"containers": []}
+        if api == "SYNO.Docker.Image" and method == "delete":
+            return {}
+        return {}
+
+    client.request.side_effect = mock_request
+    mcp, tools = make_mock_mcp()
+    register_images(mcp, make_config(), client)
+
+    result = await tools["delete_image"](
+        image_id="ghcr.io/open-webui/open-webui:v0.8.10", confirmed=True
+    )
+    assert "Deleted" in result
+    assert "open-webui" in result
+
+    # Verify delete was called with correct split params (not "ghcr" as name)
+    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"
+
+
 @pytest.mark.asyncio
 async def test_delete_image_api_error():
     from mcp_synology_container.dsm_client import SynologyError