feat: preserve UTC 'Z' indicator in Period JSON serialization

[bastien-roucaries]

When serializing PERIOD values to JSON (for jCal format), preserve the 'Z' timezone indicator for UTC datetimes. This ensures FREEBUSY periods maintain explicit UTC designation, avoiding timezone ambiguity.

Without this change:
FREEBUSY:20120226T230000Z/20120226T230000Z
=> serializes to: ["2012-02-26T23:00:00", "2012-02-26T23:00:00"]

With this change:
FREEBUSY:20120226T230000Z/20120226T230000Z
=> serializes to: ["2012-02-26T23:00:00Z", "2012-02-26T23:00:00Z"]

This behavior is consistent with RFC 5545 which requires UTC times to be designated with the 'Z' suffix.

Related: #411

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.66%. Comparing base (6afc39c) to head (d638952).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #765   +/-   ##
=========================================
  Coverage     98.66%   98.66%           
- Complexity     1757     1759    +2     
=========================================
  Files            71       71           
  Lines          5163     5165    +2     
=========================================
+ Hits           5094     5096    +2     
  Misses           69       69           

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[phil-davis]

@staabm @Chris53897 @bastien-roucaries I adjusted the code to append the "Z" when the input ends with "Z" (rather than just contains "Z"). I think that the only valid place that "Z" can be is at the end of a date-time string.

I added some test cases to DateTime testValidate to check that things like 20260627T100000Z+0200 are already being seen as invalid.

Please review and give your opinion.

[bastien-roucaries]

@phil-davis I think iut is safe