Fix duplicate Google Calendar events when completing recurring instances

[christenbc]

Summary

• Fixes duplicate Google Calendar events when completing or skipping a scheduled-anchored recurring task through the edit modal completions calendar.
• When an instance completion advances scheduled to the next occurrence, that update is no longer treated as a manual occurrence move, so googleCalendarExceptionOriginalScheduled is not set and sync no longer creates a detached exception event on top of the recurring series.
• Preserves the existing [Bug]: Google Calendar export ignores independently rescheduled next occurrence for recurring tasks #1696 moved-occurrence behavior for genuine manual scheduled reschedules.
• Adds regression coverage in taskUpdatePlanning and issue-1696-gcal-recurring-reschedule tests.

Test plan

• npx jest tests/unit/services/taskUpdatePlanning.test.ts
• npx jest tests/unit/issues/issue-1696-gcal-recurring-reschedule.test.ts
• npm run lint
• npm run build:test and reload plugin in Obsidian
• Create/sync a scheduled-anchored recurring task to Google Calendar
• Complete an instance from the edit modal completions calendar
• Confirm the next occurrence updates in TaskNotes without creating a duplicate Google event
• Manually reschedule one occurrence to another day and confirm the moved-occurrence exception behavior still works