refactor(calendar-sync-source): drop dead fetch variant, test the sentinel

The buffer-string calendar-sync--fetch-ics had no callers. The sync path uses the temp-file variant exclusively, so 30 lines of curl and sentinel logic that would only drift are gone. I also pulled the fetch-ics-file sentinel's finish logic into calendar-sync--fetch-sentinel-finish so its success, failure, and temp-file-cleanup branches can be tested without a live curl process. The async-worker tests stub the whole fetch and never reached them.

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