reject non-ASCII hex characters in Conversion.hexDigitToInt

[alhudz]

Repro: Conversion.hexToInt("ａａ", 0, 0, 0, 2) (two U+FF41 FULLWIDTH LATIN SMALL LETTER A) returns 170, the same value as the ASCII "aa", and Conversion.hexDigitToInt('٩') (U+0669 ARABIC-INDIC DIGIT NINE) returns 9. Both should throw.
Cause: hexDigitToInt reads the digit with Character.digit(hexChar, 16), which by its own specification recognises every Unicode decimal digit and the fullwidth Latin letters U+FF21-U+FF3A / U+FF41-U+FF5A, not only 0-9a-fA-F. The sibling converters hexDigitMsb0ToInt, hexDigitToBinary and hexDigitMsb0ToBinary switch over ASCII and do throw on the same chars, and CharUtils.isHex agrees with them. Since hexDigitToInt is the digit decoder behind hexToByte, hexToShort, hexToInt and hexToLong, the whole hex-string surface accepts homoglyphs, and strings that differ collapse onto one value.
Fix: screen the char with CharUtils.isHex before converting, reusing the message the siblings already use. Character.digit occurs once in the library, so this is the only site; ASCII input is unchanged.

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