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.

runtime/lua/vim/lsp/rpc.lua

@@ -323,7 +323,7 @@ end
 --- @package
 --- @param body string
 function Client:handle_body(body)
-  local ok, decoded = pcall(vim.json.decode, body, { luanil = { object = true } })
+  local ok, decoded = pcall(vim.json.decode, body)
   if not ok then
     self:on_error(M.client_errors.INVALID_SERVER_JSON, decoded)
     return
@@ -355,7 +355,6 @@ function Client:handle_body(body)
           )
         end
         if err then
-          ---@cast err lsp.ResponseError
           assert(
             type(err) == 'table',
             'err must be a table. Use rpc_response_error to help format errors.'
@@ -374,8 +373,16 @@ function Client:handle_body(body)
       end
       self:send_response(decoded.id, err, result)
     end))
-    -- This works because we are expecting vim.NIL here
-  elseif decoded.id and (decoded.result ~= vim.NIL or decoded.error ~= vim.NIL) then
+  -- Proceed only if exactly one of 'result' or 'error' is present, as required by the LSP spec:
+  -- - If 'error' is nil, then 'result' must be present.
+  -- - If 'result' is nil, then 'error' must be present (and not vim.NIL).
+  elseif
+    decoded.id
+    and (
+      (decoded.error == nil and decoded.result ~= nil)
+      or (decoded.result == nil and decoded.error ~= nil and decoded.error ~= vim.NIL)
+    )
+  then
     -- We sent a number, so we expect a number.
     local result_id = assert(tonumber(decoded.id), 'response id must be a number') --[[@as integer]]
 
@@ -415,7 +422,7 @@ function Client:handle_body(body)
         M.client_errors.SERVER_RESULT_CALLBACK_ERROR,
         callback,
         decoded.error,
-        decoded.result
+        decoded.result ~= vim.NIL and decoded.result or nil
       )
     else
       self:on_error(M.client_errors.NO_RESULT_CALLBACK_FOUND, decoded)