Photos pass size 0 (unknown, not available in InputPhotoFileLocation);
the guard then answered the first request at offset 0 with an empty
chunk, silently saving every photo as a 0-byte file.
Fixes#238
gotd's downloader is size-unaware: when a file's size is an exact
multiple of the 1MiB part size, it issues one final upload.getFile
at offset == size, which Telegram rejects with 400 OFFSET_INVALID,
failing the whole download. Wrap the client to answer such requests
with an empty chunk, matching the EOF semantics the downloader
expects. Regression test uses a fake client with real server
behavior (OFFSET_INVALID past EOF).
krau