AI plan adjustments create duplicate race calendar events instead of updating existing ones

Description:
When asking the AI coach to adjust a multi-week training plan block leading into an already-scheduled A-race, the plan-editing tool staged multiple duplicate race events on race day rather than updating or retaining the single existing event.

Steps to Reproduce:

1. Have an active training plan with a single Priority A race event already scheduled on the target date (e.g., Austin Rattler 60 on Nov 14).

2. Ask the AI coach in chat to modify workouts, volume, and taper leading up to the race.

3. The generated calendar preview added new race event entries (including variations with different placeholder durations) on top of the existing race instead of cleanly updating the single existing target event.

4. Subsequent attempts to ask the AI to remove the duplicates caused additional race entries to accumulate on the calendar/Intervals sync.

Expected Behavior:

• The plan modification patch should detect existing race/event IDs on the target date and update or preserve them rather than generating new duplicate calendar event entries.

• If a user requests removing a duplicate race, the tool should target and delete the orphaned event IDs instead of appending new entries.