turtle: TNavigator.degrees(0) raises ZeroDivisionError instead of a clear error

Bug report

turtle.TNavigator.degrees(fullcircle) — the public Turtle.degrees / turtle.degrees — divides
360 / fullcircle in _setDegreesPerAU without checking fullcircle, so passing 0 raises a
ZeroDivisionError that leaks the implementation rather than reporting the invalid argument:

>>> import turtle
>>> turtle.TNavigator().degrees(0)
Traceback (most recent call last):
  ...
ZeroDivisionError: division by zero

degrees documents fullcircle only as "a number", and 0 reaches the division directly through the
public API. It should raise a clear ValueError instead. The companion radians() passes math.tau,
so it is unaffected.

CPython versions tested on:

3.12, 3.14 (main)

Operating systems tested on:

Linux

Linked PRs

• gh-152038

從 turtle.TNavigator.degrees 和報告中描述的 _setDegreesPerAU 路徑開始，接著執行提供的 turtle.TNavigator().degrees(0) 重現。完成的標準是：透過公開的 degrees API 拒絕零，並明確拋出 ValueError 而不是 ZeroDivisionError；檢查配套的 radians() 行為仍不受影響。