fix: guard vim.lsp.diagnostics.refresh() against nil

Neovim 0.12+ restructured LSP internals; vim.lsp.diagnostics no
longer exists as a submodule. Guard the call so :SCLRescanWorkspaceTypes
doesn't crash on startup.

lua/tia_lsp/init.lua

@@ -338,7 +338,9 @@ function M.rescan_workspace_types()
 		end, 0)
 	end
 
-	vim.lsp.diagnostics.refresh()
+	if vim.lsp.diagnostics then
+		vim.lsp.diagnostics.refresh()
+	end
 end
 
 function M.create_commands()