Migrate custom item v1 capitalized item names into lowercase

[onebeastchris]

Resolves #6193

[Copilot]

Pull request overview

This PR fixes an IllegalArgumentException that occurs when custom items with uppercase letters in their names are registered via the deprecated v1 API. The issue was reported in #6193 where the Slimefun Geyser Extension was using item names like "BASE" which violated Minecraft's identifier format requirements ([a-z0-9_-./]+).

Changes:

• Automatically converts v1 custom item names to lowercase when creating item definitions to ensure identifier validity

Comments suppressed due to low confidence (1)

core/src/main/java/org/geysermc/geyser/item/GeyserCustomItemData.java:148

• Consider adding a warning when the item name contains uppercase letters that are being automatically converted to lowercase. This would help extension developers understand that their item names should be lowercase and update their code accordingly. The warning could be similar to the existing warnings for deprecated features like textureSize and renderOffsets (lines 278-286).

        GeyserCustomItemDefinition.Builder definition = (GeyserCustomItemDefinition.Builder) CustomItemDefinition.builder(Identifier.of("geyser_custom", name().toLowerCase(Locale.ROOT)), javaItem)

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