Relax document processed assertions for SG_TEST_USE_GSI=true

[torcolvin]

Reproducible running this test in a loop.

See:

sync_gateway/db/background_mgr_attachment_migration.go

Lines 133 to 144 in d20b778

	a.docsProcessed.Add(1)
	_, syncData, err := UnmarshalDocumentSyncDataFromFeed(event.Value, event.DataType, collection.userXattrKey(), false)
	if err != nil {
	base.WarnfCtx(ctx, "[%s] error unmarshaling document %s: %v, stopping attachment migration.", migrationLoggingID, base.UD(docID), err)
	a.docsFailed.Add(1)
	return false
	}
	if syncData == nil || syncData.AttachmentsPre4dot0 == nil {
	// no attachments to migrate
	return true
	}

[Copilot]

Pull Request Overview

This PR modifies an attachment migration test to handle flaky behavior when using GSI test buckets. The change relaxes a strict assertion to account for potential additional documents that may be processed due to timing issues with document mutations and deletions across snapshot boundaries.

• Updates the test assertion to be more flexible when GSI test buckets are used
• Changes from exact equality check to minimum threshold check for documents processed