* refactor(req): drop redundant error return from Strings
The error from Strings carried no information beyond emptiness — it fired
exactly when the param was absent — and nearly every caller discarded it with
a blank identifier. Strings now just returns the values (empty when absent),
making the common optional-list reads one clean expression.
The few required-param callers (scrobble, createShare) check for emptiness and
return the same Subsonic error code 10 as before; their e2e tests now pin that
code. Ints and Times keep their contracts by synthesizing ErrMissingParam
themselves, so selectedMusicFolderIds is untouched. The jellyfin parseFields
helper is inlined away, since ParseFields(p.Strings("fields")...) now
compiles directly.
* docs(req): clarify Strings returns nil when param is absent
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