User allowed mod validation is case-sensitive while playlist item allowed mods is case-insensitive

[ILW8]

Related: #406

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Title is a bit convoluted, but the gist is that ValidateUserMods in MultiplayerPlaylistItemExtensions.cs uses case-sensitive string comparison when checking user-selected mods against a playlist item's AllowedMods

osu-server-spectator/osu.Server.Spectator/Extensions/MultiplayerPlaylistItemExtensions.cs

Line 37 in 7bab117

if (item.AllowedMods.All(m => m.Acronym != mod.Acronym))

However, EditCurrentPlaylistItem accepts both upper-case and lower-case mod acronyms for AllowedMods. A referee client can to update a playlist item using lower-case mod acronyms and the game client will show the correct list of allowed mods, but the server will reject users setting any mods as user mod validation fails.

2026-04-01.05-16-59.h264_cq22.mp4

[peppy]

Probably should be case insensitive always.