feat: MongoDB outbox repository (NetEvolve.Pulse.MongoDB)

[Copilot]

Adds NetEvolve.Pulse.MongoDB — a MongoDB-backed IOutboxRepository for teams using MongoDB as their primary data store, enabling the outbox to coexist in the same database as domain aggregates without introducing a relational dependency.

New Package

• MongoDbOutboxRepository : IOutboxRepository — full async MongoDB driver implementation. Pending/failed message claiming uses FindOneAndUpdateAsync in a loop (sorted by CreatedAt) for atomic, concurrent-safe batch acquisition.
• MongoDbOutboxOptions — DatabaseName (required) and CollectionName (default: outbox_messages).
• MongoDbExtensions — two registration patterns on IMediatorBuilder:
  • AddMongoDbOutbox(...) — registers core outbox services + MongoDB repository in one call.
  • UseMongoDbOutbox(...) — provider-swap for when AddOutbox() was already called.
• IMongoClient must be pre-registered in DI by the caller.
• Health check implemented via MongoDB ping command.
• OutboxDocument maps OutboxMessage fields via [BsonId]/[BsonElement]; DateTimeOffset stored as UTC DateTime to avoid BSON representation ambiguity.

Usage

// Register IMongoClient first (caller's responsibility)
services.AddSingleton<IMongoClient>(new MongoClient(connectionString));

services.AddPulse(config => config
    .AddMongoDbOutbox(opts =>
    {
        opts.DatabaseName = "myapp";
        opts.CollectionName = "outbox_messages"; // optional, this is the default
    }));

Infrastructure

• MongoDB.Driver 3.0.0 added to Directory.Packages.props.
• Testcontainers.MongoDb 4.11.0 added for integration tests.
• DatabaseType.MongoDB added to the shared enum.
• Unit tests cover constructor validation and DI registration (avoids Mock.Of<IMongoClient>() due to TUnit.Mocks source-generator limitations with MongoDB.Driver's complex generic interfaces — uses new MongoClient(...) directly instead).
• Integration tests use MongoDbContainerFixture (shared Testcontainers instance) with per-test collection isolation via OutboxOptions.TableName → MongoDbOutboxOptions.CollectionName bridging.