This appears to be a problem with Apple macOS, not Photoshop.
The proof? If you try all the usual Mac-standard ways to expand a Save As dialog box, in non-Adobe applications and in applications that Apple includes with the Mac, none of those make the filename field wider. So the problem is with macOS itself. Shown below is what I tried in Apple Preview, and the same thing also happens in Apple Safari. I can make the dialog box wider, but the filename field doesn’t change. So it’s the way Apple is doing things.
You might already know this, but a shortcut to going to the end of the filename is to press the down arrow key.