fix: recurrence rule truncation on nth occurrence save

[SebastianKrupinski]

Summary

• added logic to prevent series truncation when editing a nth occurrence and saving to series

Issue

@nextcloud/calendar-js's forkItem() (used whenever you load any occurrence for editing) mutates the forked occurrence's own copy of RRULE, it recalculates COUNT down to "occurrences remaining from this point forward." That's correct for the "this and future" truncate flow, but the same forked object is used in series-scope save, this incorrectly truncates the series

Testing

• Without this PR
• Create a recurring daily event with a count of 5
• Edit the middle event instance (3rd)
• Save using "Update Series"
• Preform the same test with PR
• Preform test on primary occurrence by changing the recurrence count, this should properly adjust the length of the series
• Preform test on nth occurrence by changing the count and saving with "this and future", this should create a continuation with the changed count

[SebastianKrupinski]

/backport to stable6.6

[SebastianKrupinski]

/backport to stable6.5

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/store/calendarObjectInstance.js	85.71%	1 Missing ⚠️

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