Update InsertOOOEvents to collect errors

[tamaradement]

PR Description

• Updates InsertOOOEvents to collect and return errors instead of silently skipping invalid cases.
• Adds tests to verify that errors are collected for each invalid request and the returned error is a joined multi-error.

Note: Test fixtures intentionally trigger early validation failures, so no gCal mocking required.

Test steps

• Run go test ./core -v