Splits the transport-agnostic cores out of the v1 echo handlers so the
v2 Huma endpoints can share them:
- AuthenticateFeedToken(s, username, password) holds the token
validation (prefix/length guard, owner match, feeds scope, bot
rejection); BasicAuth now creates the session and delegates to it.
- BuildNotificationsAtomFeed(s, u) renders the Atom XML;
NotificationsAtomFeed reads the context user and delegates to it.
- AtomContentType is shared so both transports set the same header.
The v1 handlers keep identical observable behavior.
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