Fix the minor unit formatter for a payments API

Your payments API sends amounts as 64-bit integers in minor units (cents for USD), written as strings. An AI's serializer did Number(t.amountMinor) / 100, and after a release the mobile app showed wrong amounts for large transactions. The new formatter builds the display string without dividing.

An assistant wrote format_minor_units(amounts_minor, exponent). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• amounts_minor: amounts in minor units, as strings, such as "1999".
• exponent: how many minor digits the currency has: 2 for USD, 0 for JPY, 3 for KWD.
• Return one display string per amount, in order, or [] when exponent is below 0.

Rules the tests check:

• A valid amount is an optional - followed by one or more digits from 0 to 9. Anything else gives "invalid" for that entry.
• The result must be exact for every digit, however long the amount is.
• Drop leading zeros. Put a decimal point exponent digits from the right, and pad with zeros on the left so at least one digit comes before the point. With an exponent of 0 there is no point.
• A negative amount gets a - in front, unless it is zero: "-0" gives "0.00".