Make artifact backfill fixture a structurally valid docx

Why: The backfill contract test used a fake PK-magic docx as its fixture;
the new registration structural gate correctly refuses that shape, so the
contract was no longer exercised.

Evidence: The three affected runmgr tests pass locally with -count=1;
the full combined suite receipt follows on the task card.

Impact: Test fixture only; the gate behavior under test is unchanged.

Refs: Dev task feature/pptx-register-structural-gate

internal/runmgr/exec_tool_artifact_test.go

@@ -77,9 +77,11 @@ func TestRunPythonRegisterArtifactBackfill(t *testing.T) {
 	if err := tools.RegisterInterpreterTools(c.mgr.registry); err != nil {
 		t.Fatal(err)
 	}
+	// 夹具必须是结构合法的最小 docx：登记口会做 OPC 结构自检，
+	// 伪 zip（PK 魔数+垃圾字节）现在会被拒绝而非登记。
 	py := c.ExecuteTool(core.ToolCallRequest{
 		CallID: "py1", Name: "run_python",
-		Arguments: json.RawMessage(`{"script":"open('note.docx','wb').write(b'PK\\x03\\x04hello')\n"}`),
+		Arguments: json.RawMessage(`{"script":"import zipfile\nwith zipfile.ZipFile('note.docx','w') as z:\n    z.writestr('[Content_Types].xml','<?xml version=\"1.0\"?><Types/>')\n    z.writestr('word/document.xml','<?xml version=\"1.0\"?><doc/>')\n"}`),
 	})
 	if py.Status != "ok" || !strings.Contains(py.Output, "exit=0") {
 		t.Fatalf("run_python: %s %s", py.Status, py.Output)