Reject over-length codes in ISSNCheckDigit

[sahvx655-wq]

Noticed while exercising the check digit routines directly rather than through their wrapper validators.

1. ISSNCheckDigit.ISSN_CHECK_DIGIT.isValid is a public entry point and ModulusCheckDigit never checks the code length, so an over-length string reaches the modulus test unchecked.
2. weightedValue weights each position by 9 - leftPos, so a ninth character lands on weight zero and contributes nothing; appending any character to a valid numeric ISSN (031784710..031784719, 0317847100) still returns true, though only the eight-character 03178471 is a real ISSN.
3. Guarded isValid to require the fixed eight-character length before the check digit test, the same shape as the existing IBANCheckDigit length guard; the ISSNValidator/CodeValidator path already enforces length, so valid input is unaffected.

Regression test testOverLengthRejected added, which fails on the current code and passes with the guard.

[garydgregory]

@sahvx655-wq
See my comment #423 (review)

[garydgregory]

Thank you @sahvx655-wq , merged 🚀 Please check for other length issues in this package.

[sahvx655-wq]

Will do. From a quick look the mirror form exists in the rightPos-weighted routines: ISBN10CheckDigit and ABANumberCheckDigit have no length check either, and there a prepended zero is the character that lands on a no-op weight, so a leading-zero-padded code keeps the same sum. I'll work through the package and put up fixes for the ones reachable outside their wrapper validators. Thanks for merging this.