fix(lsp): decode 'null' in server responses as vim.NIL #34849

Problem:
Previously, 'null' value in LSP responses were decoded as 'nil'.
This caused ambiguity for fields typed as '? | null' and led to
loss of explicit 'null' values, particularly in 'data' parameters.

Solution:
Decode all JSON 'null' values as 'vim.NIL' and adjust handling
where needed. This better aligns with the LSP specification,
where 'null' and absent fields are distinct, and 'null' should
not be used to represent missing values.

This also enables proper validation of response messages to
ensure that exactly one of 'result' or 'error' is present, as
required by the JSON-RPC specification.

src/gen/gen_lsp.lua

@@ -370,7 +370,7 @@ local function write_to_meta_protocol(protocol, version, output_file)
     '---@meta',
     "error('Cannot require a meta file')",
     '',
-    '---@alias lsp.null nil',
+    '---@alias lsp.null vim.NIL',
     '---@alias uinteger integer',
     '---@alias decimal number',
     '---@alias lsp.DocumentUri string',