Aligns update_entire_series with the same metadata preservation approach
used in this_only and this_and_future update methods.
## Key Changes:
1. **Clone-and-Modify Pattern**:
- Clone existing event to preserve all metadata (organizer, attendees, etc.)
- Only modify specific properties that need to change
- Maintains consistency with other update methods
2. **Smart Field Handling**:
- Preserve original values when request fields are empty
- Only overwrite when new values are explicitly provided
- Same selective update logic as other scopes
3. **RRULE Preservation**:
- Keep existing recurrence pattern unchanged for simple updates
- Suitable for drag operations that just change start/end times
- Avoids breaking complex RRULE patterns unnecessarily
4. **Proper Timestamp Management**:
- Update dtstamp and last_modified to current time
- Preserve original created timestamp for event history
- Consistent timestamp handling across all update types
## Benefits:
- All three update scopes now follow the same metadata preservation pattern
- Simple time changes (drag operations) work without side effects
- Complex event properties maintained across all modification types
- Better RFC 5545 compliance through proper event structure preservation
## Removed:
- Complex RRULE regeneration logic (build_series_rrule function now unused)
- Manual field-by-field assignment replaced with selective clone modification
This ensures consistent behavior whether users modify single occurrences,
future events, or entire series - all maintain original event metadata.
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
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