feat: Azure Cosmos DB outbox repository (NetEvolve.Pulse.CosmosDb)

[Copilot]

Adds NetEvolve.Pulse.CosmosDb, a native Cosmos DB persistence provider for the outbox pattern, allowing teams using document-oriented architectures on Azure Cosmos DB to avoid introducing a relational database solely for outbox support.

New package: NetEvolve.Pulse.CosmosDb

• CosmosDbOutboxOptions — configures DatabaseName, ContainerName (default outbox_messages), PartitionKeyPath (default /id), EnableTimeToLive, and TtlSeconds (default 86400)
• CosmosDbOutboxRepository — full IOutboxRepository implementation using async Cosmos SDK APIs; pending-message claim uses PatchItemAsync with IfMatchEtag for ETag-based optimistic concurrency to prevent duplicate processing
• CosmosDbOutboxManagement — full IOutboxManagement implementation for dead-letter inspection, replay, and statistics queries
• CosmosDbExtensions — AddCosmosDbOutbox (registers core outbox + repository) and UseCosmosDbOutbox (repository-only swap) on IMediatorBuilder; CosmosClient must be pre-registered by the caller

Usage

// Register CosmosClient first
services.AddSingleton(new CosmosClient(connectionString, new CosmosClientOptions
{
    Serializer = new CosmosSystemTextJsonSerializer(JsonSerializerOptions.Default)
}));

services.AddPulse(c => c.AddCosmosDbOutbox(opts =>
{
    opts.DatabaseName  = "MyDatabase";
    opts.ContainerName = "outbox_messages";   // default
    opts.EnableTimeToLive = true;             // auto-expire completed/dead-letter docs
    opts.TtlSeconds = 86400;                  // 24 h (default)
}));

Note: The container and database must exist prior to use — automatic schema creation is out of scope. The CosmosClient should be configured with CosmosSystemTextJsonSerializer to align with the System.Text.Json attributes used on the internal document model.

Infrastructure changes

• Microsoft.Azure.Cosmos 3.58.0 added to Directory.Packages.props
• Project added to Pulse.slnx and referenced from the unit test project

Known gaps in this PR

• Unit tests for extensions and repository not yet written
• Integration tests using Testcontainers Cosmos DB emulator not implemented