Return a Float from Money#/ for a plain Money divisor on a subclass

[andrewstellman]

Repro, with library defaults (no global settings changed):

Sub = Class.new(Money)
Sub.new(1000, "USD") / Money.new(100, "USD")
# actual:   #<Sub fractional:1000 currency:USD>
# expected: 10.0

Money.new(1000, "USD") / Money.new(100, "USD") returns 10.0. With a zero plain Money divisor, the subclass raised divided by zero instead of divided by Money(0).

For a subclass receiver and a divisor in another currency, the old code did not convert the divisor. It used the divisor's to_d as a plain number, so Sub.new(1000, "USD") / Money.new(400, "GBP") returned #<Sub fractional:250 currency:USD>. The new code converts the divisor through the bank as Money / Money does, so a missing rate raises Money::Bank::UnknownRate.

Cause: Money#/ tests the divisor with other.is_a?(self.class) (lib/money/money/arithmetic.rb:205). For a subclass receiver, a divisor that is not an instance of that same subclass (a plain Money, or another subclass) takes the number branch. The other Money checks in that file use is_a?(Money) (lines 41, 66, 244, 302).

The new spec fails without the change and passes with it. Full suite: 503 examples, 0 failures.

The method docs say: "+Money/Numeric+ returns +Money+. +Money/Money+ returns +Float+." (lib/money/money/arithmetic.rb:193).

Found by Quality Playbook, an AI code-review tool, with Claude; I reviewed the change.