Capture domain events and validate outbox delay options for sync SaveChanges callers

[Vulthil]

Summary

• DomainEventsToOutboxMessageSaveChangesInterceptor now also overrides the synchronous SavingChanges/SavedChanges EF Core hooks (previously only the async ones), so a bare synchronous SaveChanges() call captures domain events and wakes the relay identically to the async path instead of silently dropping them.
• AddOutboxEngine now validates that MaxDelaySeconds is greater than or equal to OutboxProcessingDelaySeconds at startup, so a contradictory configuration (e.g. base delay 100s, max 1s) fails fast instead of silently clamping every poll delay to the max.

Verification

• dotnet build on the full solution: 0 errors, PublicAPI analyzers green.
• Full test runs (all green): Vulthil.SharedKernel.Outbox.Tests, Vulthil.SharedKernel.Outbox.EntityFrameworkCore.Tests.
• Verified both new tests fail against the pre-fix code and pass once the fix is restored.

Backport to v1.0: no — adds public API (patch-ineligible).