fix(api): gate sandbox artifact download on agent session ownership (#16169)

Fixes #16168

## Summary
- Add session-scoped authorization for `GET
/api/v1/documents/artifact/<filename>`
- Allow download only when the artifact filename appears in the caller's
`api_4_conversation` message and
`UserCanvasService.accessible(dialog_id, user_id)` passes
- Deny with generic `"Artifact not found."` before storage access (no
cross-user enumeration)
- Return 4xx when the blob is missing (existing behavior preserved)

## Approach
Sandbox artifacts are runtime CodeExec outputs, not KB documents — this
uses the same session gate pattern as `agent_chat_completion`, not
`DocumentService.accessible`.

## Test plan
- [x] Unit: denied when filename not referenced in user sessions
- [x] Unit: denied when agent canvas is not accessible
- [x] Unit: authorized user receives bytes; missing blob returns
`"Artifact not found."`
- [ ] `pytest
test/testcases/test_web_api/test_document_app/test_document_metadata.py
-k get_artifact`

---------

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Zhichang Yu <yuzhichang@gmail.com>

test/testcases/test_web_api/test_document_app/test_document_metadata.py

@@ -641,6 +641,79 @@ class TestDocumentMetadataUnit:
         assert res["code"] == RetCode.DATA_ERROR
         assert "Image not found" in res["message"]
 
+    @pytest.mark.p2
+    def test_get_artifact_denied_without_session_reference_unit(self, document_app_module, monkeypatch):
+        module = document_app_module
+        filename = "a1b2c3d4e5f6789012345678901234abcd.png"
+
+        monkeypatch.setattr(module, "_sandbox_artifact_dialog_ids_for_user", lambda *_args, **_kwargs: [])
+        res = _run(module.get_artifact(filename))
+        assert res["code"] == RetCode.DATA_ERROR
+        assert res["message"] == "Artifact not found."
+
+    @pytest.mark.p2
+    def test_get_artifact_denied_when_agent_not_accessible_unit(self, document_app_module, monkeypatch):
+        module = document_app_module
+        filename = "a1b2c3d4e5f6789012345678901234abcd.png"
+
+        monkeypatch.setattr(module, "_sandbox_artifact_dialog_ids_for_user", lambda *_args, **_kwargs: ["agent-1"])
+        monkeypatch.setattr(module.UserCanvasService, "accessible", lambda *_args, **_kwargs: False)
+        res = _run(module.get_artifact(filename))
+        assert res["code"] == RetCode.DATA_ERROR
+        assert res["message"] == "Artifact not found."
+
+    @pytest.mark.p2
+    def test_get_artifact_success_and_missing_blob_unit(self, document_app_module, monkeypatch):
+        module = document_app_module
+        filename = "a1b2c3d4e5f6789012345678901234abcd.png"
+
+        class _Headers(dict):
+            def set(self, key, value):
+                self[key] = value
+
+        class _ArtifactResponse:
+            def __init__(self, data):
+                self.data = data
+                self.headers = _Headers()
+
+        monkeypatch.setattr(module, "_sandbox_artifact_dialog_ids_for_user", lambda *_args, **_kwargs: ["agent-1"])
+        monkeypatch.setattr(module.UserCanvasService, "accessible", lambda *_args, **_kwargs: True)
+
+        async def fake_thread_pool_exec(fn, *args, **kwargs):
+            return fn(*args, **kwargs)
+
+        async def fake_make_response(data):
+            return _ArtifactResponse(data)
+
+        monkeypatch.setattr(module, "thread_pool_exec", fake_thread_pool_exec)
+        monkeypatch.setattr(module, "make_response", fake_make_response)
+        monkeypatch.setattr(
+            module,
+            "apply_safe_file_response_headers",
+            lambda response, content_type, extension: response.headers.update(
+                {"content_type": content_type, "extension": extension}
+            ),
+        )
+        monkeypatch.setattr(
+            module.settings,
+            "STORAGE_IMPL",
+            SimpleNamespace(get=lambda *_args, **_kwargs: b"artifact-bytes"),
+        )
+
+        res = _run(module.get_artifact(filename))
+        assert isinstance(res, _ArtifactResponse)
+        assert res.data == b"artifact-bytes"
+        assert res.headers["content_type"] == "image/png"
+
+        monkeypatch.setattr(
+            module.settings,
+            "STORAGE_IMPL",
+            SimpleNamespace(get=lambda *_args, **_kwargs: None),
+        )
+        res = _run(module.get_artifact(filename))
+        assert res["code"] == RetCode.DATA_ERROR
+        assert res["message"] == "Artifact not found."
+
 
 class TestDocumentBatchChangeStatus:
     @pytest.mark.p2