Improved configuration validation

When EzMine loads custom-tools it performs sanity checks and reports problems in the server log. Fatal errors (such as a missing or unknown material, or duplicate tool IDs after normalization) will disable custom-tools until you fix them.

Watch for these common messages and how to fix them:

• Missing material for custom tool <id> — add a valid Spigot Material name (e.g. NETHERITE_PICKAXE).
• Unknown material <name> for custom tool <id> — correct the material name or use a supported material.
• Duplicate custom tool id after normalization: <id> — ensure tool keys are unique when lowercased.
• Unknown action '<action>' for custom tool <id> — use a built-in action (e.g. 3x3, auto-smelt, ore-searcher) or define it in the actions section.
• custom-model-data must be zero or positive for custom tool <id> — use a non-negative integer for custom-model-data.
• shop.slot out of range for custom tool <id> — choose a slot between 0 and rows*9 - 1.

If you see warnings (non-fatal issues), EzMine will still load the tool but the behaviour may differ. After fixing the YAML, run /ezmine reload to re-apply the configuration.