Fix iso6346 malformed strings

[SarthakDudhe]

The fix for Issue #2772 is complete and verified!
Grouped and anchored the alternation in isISO6346Str regex in
isISO6346.js
to prevent matching malformed strings with leading/trailing characters.
Corrected the character set from [J,Z] to [JZ] to remove the literal comma match.
Added comprehensive invalid tests for both isISO6346 and isFreightContainerID in
validators.test.js
and verified that all 318 tests pass successfully.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a38f15b) to head (3c1354c).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2797   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2587      2587           
  Branches       656       656           
=========================================
  Hits          2587      2587           

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