Return a copy projected to the negotiated protocol version. Per-content _meta was introduced by 2025-06-18 ("Add _meta to additional interface types"), so it is dropped for an older peer. uri/mimeType/text/blob all existed in 2024-11-05 and are preserved unchanged. Mirrors the content-level _meta stripping in Content.forVersion.
optional _meta object
The contents of a resource read: either UTF-8 text or base64 blob.