fix(lsp): retrigger diagnostics request on server cancellation (#31345)

Co-authored-by: Jesse <github@jessebakker.com>

runtime/lua/vim/lsp/diagnostic.lua

@@ -246,10 +246,18 @@ end
 ---
 --- See |vim.diagnostic.config()| for configuration options.
 ---
----@param _ lsp.ResponseError?
+---@param error lsp.ResponseError?
 ---@param result lsp.DocumentDiagnosticReport
 ---@param ctx lsp.HandlerContext
-function M.on_diagnostic(_, result, ctx)
+function M.on_diagnostic(error, result, ctx)
+  if error ~= nil and error.code == protocol.ErrorCodes.ServerCancelled then
+    if error.data == nil or error.data.retriggerRequest ~= false then
+      local client = assert(vim.lsp.get_client_by_id(ctx.client_id))
+      client:request(ctx.method, ctx.params)
+    end
+    return
+  end
+
   if result == nil or result.kind == 'unchanged' then
     return
   end

runtime/lua/vim/lsp/handlers.lua

@@ -659,7 +659,8 @@ for k, fn in pairs(M) do
       })
     end
 
-    if err then
+    -- ServerCancelled errors should be propagated to the request handler
+    if err and err.code ~= protocol.ErrorCodes.ServerCancelled then
       -- LSP spec:
       -- interface ResponseError:
       --  code: integer;

runtime/lua/vim/lsp/protocol.lua

@@ -174,6 +174,7 @@ local constants = {
     -- Defined by the protocol.
     RequestCancelled = -32800,
     ContentModified = -32801,
+    ServerCancelled = -32802,
   },
 
   -- Describes the content type that a client supports in various