Refuse a price that is a fraction of a minor unit

[kha333n]

Fixes #2689.

1.x only — 2.x has replaced this cast, and I have not checked its behaviour.

A price written as 12.99 is stored as 13, and the column counts minor units,
so the product sells for AED 0.13.

The fix

set() returned the value unchanged, so a decimal reached a bigint column and
was rounded. It now refuses a value that is not a whole number of minor units,
and says what was probably meant:

Prices are stored as a whole number of minor units, so [price] cannot be 12.99.
Did you mean 1299?

Refusing rather than converting is deliberate: the cast cannot tell whether a
bare 12 is twelve minor units or twelve of the major unit — both are in use —
and guessing would multiply every correctly-written price by the currency factor.
The documented contract is already an integer count of minor units, so this makes
the code agree with the docs rather than changing the contract.

get() is untouched.

The existing tests

PriceTest passes a decimal in four places. Correcting those inputs to the
minor-unit equivalent leaves every assertion in the file unchanged — ->value,
->decimal and ->formatted() all still hold:

line	was	now
74	12.99	1299
91	12.995 (3dp currency)	12995
153	12.99	1299
154	13.99 (compare_price)	1399

They only appeared to work because the test asserts on the in-memory model and
never reloads it — get() strips non-digits, so "12.99" reads back as 1299
in the request that wrote it, and 13 in every request after.

Tests

tests/core/Unit/Base/Casts/PriceTest.php:

• can not set a price to a fraction of a minor unit — over price and
  compare_price.
• can say what the price was probably meant to be — the message carries
  1299, so the fix is actionable rather than just a refusal.
• can set a price to a whole number of minor units — an integer, an integral
  float and a numeric string all still store 1299. Passes before and after, and
  is the guard that this rejects fractions rather than non-integers.
• can leave a null compare price alone.