Function materialize_resource_blob 

pub fn materialize_resource_blob(
    workspace_dir: &Path,
    uri: Option<&str>,
    mime_type: Option<&str>,
    blob_b64: &str,
) -> Result<MaterializedResource, EmbeddedResourceError>

Decode blob_b64, enforce size limits, write under {workspace}/uploads/, and return a prompt marker ([Document: …] or [IMAGE:…]). Thin base64 front door over materialize_bytes.