The 1.0 SDK ignores unknown part fields, so a file part shaped {kind:file, file:{bytes|uri,
name, mimeType}} arrived as an empty part and vanished. A2AWireCompat rewrites such parts
to {raw|url, filename, mediaType} in JSON and SSE responses before the SDK reads them, and
marks any part it still can't read so the result lists it with its field names.