Harden calendar auto-sync event dedupe

[thomasluizon]

Summary

• dedupe fetched Google Calendar event ids before processing auto-sync
• reserve event ids during reconciliation and suggestion creation to avoid duplicate assignments in the same sync pass
• add regression coverage for duplicate fetched event ids and clean a duplicate test using

Testing

• dotnet test C:\orbit-api\tests\Orbit.Application.Tests\Orbit.Application.Tests.csproj --filter "FullyQualifiedNameRunCalendarAutoSyncCommandHandlerTests|FullyQualifiedNameGetCalendarEventsQueryHandlerTests"
• dotnet test C:\orbit-api\tests\Orbit.Infrastructure.Tests\Orbit.Infrastructure.Tests.csproj --filter FullyQualifiedName~DistributedRateLimitServiceTests

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud