Standardize Mason plugin repository naming

[Copilot]

Resolves naming inconsistency where Mason plugins were referenced using both mason-org/ and williamboman/ namespaces, which could cause dependency resolution failures.

Changes

• Standardized all Mason plugin references to use mason-org/ namespace
• Updated mason-null-ls.nvim dependency from williamboman/mason.nvim to mason-org/mason.nvim

dependencies = {
-  "williamboman/mason.nvim",
+  "mason-org/mason.nvim",
   "nvimtools/none-ls.nvim",
   "nvim-lua/plenary.nvim",
},

All Mason-related plugins now consistently use:

• mason-org/mason.nvim
• mason-org/mason-lspconfig.nvim
• jay-babu/mason-null-ls.nvim

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