Preserve render product lists in BaseWriterNode

[sylvesterkaczmarek]

Description

Fix multi-render-product attachment handling in BaseWriterNode.

attach_writer() and attach_writers() accept either a single render product path or a list[str]. The requested value is passed correctly to the writer, but the IsaacNoop annotator unconditionally wrapped it in another list:

noop.attach([request.render_product_path])

For a multi-render-product request this produces a nested list such as [[path_a, path_b]] instead of the expected [path_a, path_b].

Normalize only scalar paths before attaching the no-op annotator, preserving an existing list unchanged.

Validation

• focused regression test covers a two-render-product request
• verifies the writer and IsaacNoop annotator receive the same flat list
• single-path behavior remains unchanged
• production change is limited to base_writer_node.py