fix: lint warning and test mocks for async file read

Problem: selene flagged unused err3 variable, and test mock_enums
still mocked io.open instead of the new vim.uv.fs_* calls.

Solution: rename err3 to _, replace io.open mock with synchronous
vim.uv.fs_open/fs_fstat/fs_read/fs_close mocks using a sentinel fd.

lua/blink-cmp-ghostty.lua

@@ -193,7 +193,7 @@ function M:get_completions(ctx, callback)
             on_all_done()
             return
           end
-          vim.uv.fs_read(fd, stat.size, 0, function(err3, data)
+          vim.uv.fs_read(fd, stat.size, 0, function(_, data)
             vim.uv.fs_close(fd)
             enums_content = data or ''
             on_all_done()