Add MongoDB transactions to WordRepository multi-collection operations

[Copilot]

WordRepository methods that coordinate writes across WordsCollection and FrontierCollection were not atomic — a failure mid-operation could leave the two collections in an inconsistent state.

Changes

• IMongoTransaction: Added IClientSessionHandle Session { get; } so callers can pass the session handle to MongoDB collection operations within a transaction
• MongoDbContext: Implemented Session in MongoTransactionWrapper
• MongoDbContextMock: Stubbed Session => null! (session is never accessed in tests; WordRepository is [ExcludeFromCodeCoverage] and tests use WordRepositoryMock)
• WordRepository: Stored _dbContext field; wrapped all 5 multi-collection methods in transactions with explicit commit/abort:
  • DeleteAllWords
  • Create(Word) / Create(List<Word>)
  • CreateAndDeleteFrontier
  • AddAndDeleteFrontier

Each method follows the pattern:

using var transaction = await _dbContext.BeginTransaction();
try
{
    await _words.InsertOneAsync(transaction.Session, word);
    await _frontier.InsertOneAsync(transaction.Session, word);
    await transaction.CommitTransactionAsync();
}
catch
{
    await transaction.AbortTransactionAsync();
    throw;
}

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

---

This change is