Add Complete() method to IAuditIngestionUnitOfWork

[rbev]

This pull request introduces a new Complete method to the IAuditIngestionUnitOfWork interface and implements it across all audit ingestion unit of work classes.

Why

As preparation work for implementing audit ingestion on entity framework, this aligns the ingestion unit of work with it's sibling for error ingestion in the Primary instance.

While the current RavenDB ingestor makes use of a BulkInsertOperation that does not provide a full transactional rollback the upcoming SQL pipeline will.

API and Interface Changes

• Added a Complete(CancellationToken cancellationToken = default) method to the IAuditIngestionUnitOfWork interface to signal when all records have been added and the unit of work should commit.
• Implemented the new Complete method in both InMemoryAuditIngestionUnitOfWork (as a no-op, since writes are immediate) and RavenAuditIngestionUnitOfWork (which flushes and disposes the bulk insert, ensuring all records are written). [1] [2]
• Updated AuditPersister to call unitOfWork.Complete() after persisting audit records, ensuring the commit point is respected.
• Refactored all test usages of audit ingestion unit of work to use await using for proper disposal and replaced explicit DisposeAsync() calls with the new Complete() method, improving resource management and consistency with the new API.