修复 External 去背景重放与画布竞态

按原始请求指纹在可变预检前返回幂等任务
修正来源身份优先级与跨项目歧义校验
冻结并复验原位替换目标的完整媒体语义
修复 Python helper 原位替换请求并补正向路由测试

.codex/skills/genarrative-external-editor-api/scripts/genarrative_external_api.py

@@ -582,11 +582,16 @@ class GenarrativeExternalClient:
         session = fields.get("canvasSession")
         if session is None:
             session = fields.get("canvas_session")
+        target_layer_id = normalize_optional_text(fields.get("targetLayerId"))
+        if target_layer_id and fields.get("canvasCompletion") is not None:
+            raise GenarrativeApiError(
+                "targetLayerId and canvasCompletion are mutually exclusive for background removal"
+            )
         canvas_width = fields.get("canvasWidth")
         canvas_height = fields.get("canvasHeight")
         if (canvas_width is None) != (canvas_height is None):
             raise GenarrativeApiError("canvasWidth and canvasHeight must be provided together")
-        if session is not None and canvas_width is None:
+        if session is not None and canvas_width is None and not target_layer_id:
             if source_width is None or source_height is None:
                 raise GenarrativeApiError(
                     "remove_background requires source_width and source_height when canvasSession is used without canvasWidth/canvasHeight"
@@ -599,6 +604,8 @@ class GenarrativeExternalClient:
             source_width or 1,
             source_height or 1,
         )
+        if target_layer_id:
+            fields.pop("canvasCompletion", None)
         idempotency_key = fields.pop("idempotencyKey", None)
         return self.submit_and_wait_generation(
             "/api/external/v1/editor/images/background-removals",
@@ -838,6 +845,30 @@ def _self_test() -> None:
     assert calls[0]["body"]["canvasCompletion"]["placeholder"]["width"] == 720
     assert calls[0]["body"]["canvasCompletion"]["placeholder"]["height"] == 1280
     calls.clear()
+    client.remove_background(
+        "uploads/source.png",
+        canvasSession=session,
+        targetLayerId="layer-1",
+        assetLabel="原位去背景结果",
+    )
+    assert calls[0]["body"]["projectId"] == "proj-demo"
+    assert calls[0]["body"]["assetFolderId"] == "editor-asset-folder-demo"
+    assert calls[0]["body"]["assetLabel"] == "原位去背景结果"
+    assert calls[0]["body"]["targetLayerId"] == "layer-1"
+    assert "canvasCompletion" not in calls[0]["body"]
+    calls.clear()
+    try:
+        client.remove_background(
+            "uploads/source.png",
+            canvasSession=session,
+            targetLayerId="layer-1",
+            canvasCompletion={"title": "冲突完成指令"},
+        )
+    except GenarrativeApiError as error:
+        assert "targetLayerId and canvasCompletion are mutually exclusive" in str(error)
+    else:
+        raise AssertionError("background removal must reject conflicting canvas placement modes")
+    assert calls == []
     try:
         client.remove_background("uploads/source.png", canvasSession=session)
     except GenarrativeApiError as error: