Skip cancelled Google Calendar events before sync matching

[Zhepyrx]

Fixes #315

Summary

• Filter existing Google Calendar events with status: "cancelled" before building the calendarEventsIds matching array.
• Keep confirmed/tentative/statusless events eligible for normal update matching.
• Log how many cancelled existing events were skipped when any are present.

Why

The sync fetches existing Google Calendar events with showDeleted: false, but issue discussion reports that cancelled events can still appear in this result set. If a cancelled event has the same extended private ID as an incoming ICS event, the sync attempts Calendar.Events.update(...) against the cancelled event and can fail with Invalid start time.

Filtering cancelled existing events before the ID/MD5 arrays are built prevents them from becoming update candidates.

Verification

node --check --input-type=commonjs < Code.gs
node --check --input-type=commonjs < Helpers.gs
git diff --check HEAD~1 HEAD
node -e 'const events=[{id:"1",status:"confirmed"},{id:"2",status:"cancelled"},{id:"3"},null,undefined,{id:"4",status:"CANCELLED"}]; const filtered=events.filter(function(event){return event != null && (event.status == null || event.status.toString().toLowerCase() != "cancelled");}); console.log(JSON.stringify(filtered.map(e => e.id))); if (JSON.stringify(filtered.map(e => e.id)) !== JSON.stringify(["1","3"])) process.exit(1);'

All commands passed locally.

[Zhepyrx]

Closing this because I found #520 after opening, and it addresses the same IssueHunt-funded issue first. My PR added a slightly more defensive filter/log, but keeping a duplicate open would add maintainer noise.