Add lightweight transactional outbox sample

[balajibashyam]

Summary

Adds a minimal .NET 10 companion sample for the Transactional Outbox Pattern in .NET with EF Core (.NET 10): Fix the Dual-Write Problem tutorial.

What's included

• New sample: distributed-systems/transactional-outbox-ef-core/ — 14 files

  • Creates an Order and OutboxMessage in one atomic SaveChangesAsync
  • Relays pending outbox rows via a one-shot OutboxRelay
  • Uses InMemoryPublisher for deterministic demonstration
  • 3 xUnit v3 tests: atomic write, relay processing, no duplicate dispatch
  • Console program with deterministic count-based output
  • SQLite (no Docker, PostgreSQL, or RabbitMQ required)

• Root README: New sample row in the table + updated repository structure

• GitHub Actions: New test-transactional-outbox job in the shared workflow; path filters updated

Deliberately omitted (stays in the full tutorial)

ASP.NET Core API endpoints, PostgreSQL, Npgsql, RabbitMQ, Docker Compose, broker SDKs, a separate consumer application, inbox tables, duplicate-consumer handling, concurrent dispatchers, FOR UPDATE SKIP LOCKED, SQL Server locking hints, SaveChangesInterceptor, aggregate domain-event infrastructure, EF Core execution-strategy demonstrations, broker retries, poison-message quarantine, dead-letter storage, exponential backoff, OpenTelemetry, trace propagation, health checks, push-then-poll signaling, channels, change data capture, Debezium, migrations, Testcontainers, production secrets, deployment, library comparisons.

Versions

• .NET SDK: 10.0.302
• Microsoft.EntityFrameworkCore: 10.0.10
• Microsoft.EntityFrameworkCore.Sqlite: 10.0.10
• xunit.v3: 3.2.2

Validation

• dotnet restore — passed
• dotnet build --configuration Release — passed (0 errors)
• dotnet test --configuration Release — passed (3/3)
• dotnet run — passed (console output matches expected)
• Security scan — clean