fix(nntp): materialize ARTICLE/HEAD/BODY multiline data to prevent em…

[ros32]

…pty bodies

The NNTP parser returned deferred IEnumerable<string> body lines backed by an enumerator created inside a using block. In some usage patterns (e.g., deferred enumeration, interleaved NNTP commands, keepalive/parallel workers) the enumerator would be disposed before consumers enumerated it, yielding empty bodies.

Change:

• In lib/Usenet/Usenet/Nntp/Parsers/ArticleResponseParser.cs (Parse):
  • Copy dataBlock to a list (var lines = dataBlock?.ToList();)
  • Enumerate headers from this list
  • Materialize body with EnumerateBodyLines(enumerator).ToList()
  • Return NntpArticle with fully materialized Body

Why now:

• Recent calling patterns defer enumeration and/or issue additional NNTP commands between parse and consumption, exposing the latent bug.

Notes:

• RFC 3977 compliance preserved; dot-stuffing and terminator remain handled by NntpStreamReader.
• No public API changes; minor memory overhead per article body in exchange for correctness.

[JohnTasler]

@ros32, I recommend moving this PR into the fork at https://github.com/Spottarr/Usenet. It is being maintained more actively, and the PR will probably be merged much more quickly than if you leave it on this original fork.