A FAILED delivery item never sets fileCount/totalBytes, and the worker's
Dictionary-based JSON payload serializes those as explicit nulls (the
WhenWritingNull option only suppresses nulls on POCO properties, not
dictionary values). The server's schema allowed the fields to be omitted
but not null, so the report was permanently rejected with 422 - and the
worker only ever treated 409/404 as terminal, so it retried the same
malformed payload forever across restarts.
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