Preserve exact value in BigDecimal and BigInteger validate

[sahvx655-wq]

Following up on the earlier range-check work I checked what validate() actually hands back for high-precision input, and both validators drop digits. BigDecimalValidator.validate("0.12345678901234567890") returns 0.12345678901234568, and BigIntegerValidator on an integer past the long range (92233720368547758077) returns 92233720368547760000. The parse runs through the inherited NumberFormat, whose default DecimalFormat returns a Double for any fractional value or any integer outside long range, so the value is already rounded to roughly 17 significant figures before it is wrapped in a BigDecimal or BigInteger. For a pair of classes whose whole purpose is exact conversion that is a real correctness problem, and it is silent: no exception, just a different value than the caller supplied.

Overriding getFormat in each validator to put the DecimalFormat into setParseBigDecimal(true) makes parseObject return a BigDecimal up front, so the exact value reaches the conversion step. It belongs at the format layer because that is where the lossy Double is created, and nothing downstream can recover digits that parsing has already discarded. The two validators are kept in step, which an existing test already pins to the same magnitude, and one fixture that encoded the rounded result has been updated to the exact value. New regression tests cover a high-precision decimal and an integer scaled up from Long.MAX_VALUE.