Test coverage: DataModel.AddManyChanges bulk write and chunking

[hahn-kev-bot]

Follow-up from the test-suite review (batches merged as #92–#96).

Gap

DataModel.AddManyChanges is a public bulk-write entry point with its own chunking (changesPerCommitMax, one commit per chunk) and empty-input early return (if (commits is []) return). No test calls it — the chunk-splitting math and per-chunk commit flow are entirely unverified.

harmony/src/SIL.Harmony/DataModel.cs

Lines 61 to 80 in 03f609f

	public async Task AddManyChanges(Guid clientId,
	IEnumerable<IChange> changes,
	Func<CommitMetadata?> commitMetadata,
	int changesPerCommitMax = 100)
	{
	await using var repo = await _crdtRepositoryFactory.CreateRepository();
	var commits = changes
	.Chunk(changesPerCommitMax)
	.Select(chunk => NewCommit(clientId, commitMetadata(), chunk))
	.ToArray();
	if (commits is []) return;
	using var locked = await repo.Lock();
	repo.ClearChangeTracker();
	await using var transaction = await repo.BeginTransactionAsync();
	var updatedCommits = await repo.AddCommits(commits);
	await UpdateSnapshots(repo, updatedCommits);
	await ValidateCommits(repo);
	await transaction.CommitAsync();
	}

Suggested tests

• AddManyChanges_SplitsChangesIntoCommitsOfMaxSize
• AddManyChanges_WithNoChangesDoesNothing

Why it matters

It is a distinct write path with its own transaction/lock/validate flow; a bug in the chunking could silently mis-group changes into the wrong commits.