feat(mediator): add optimistic concurrency to sample

[Copilot]

The mediator sample lacked optimistic-concurrency handling for concurrent greeting updates. This adds version-aware persistence and retry behavior, while documenting ETag/If-Match as a follow-up requiring generator support.

• Concurrency

  • Add SQL ROWVERSION tracking to greetings.
  • Detect stale writes and raise OptimisticConcurrencyException.
  • Mirror version checks in the in-memory store.

• Retries

  • Register a request-handler decorator that retries optimistic conflicts up to two times.
  • Propagate exhausted conflicts for standard 409 Conflict mapping.

• Coverage

  • Add retry, exhausted-conflict, and stale-version tests.
  • Mark SMP-04 complete in the mediator task board.

container.RegisterDecorator(
    typeof(IRequestHandler<,>),
    typeof(OptimisticConcurrencyRetrierDecorator<,>));

ETag/If-Match response-header support remains deferred because generated response-header emission is not yet available.

[Copilot]

Pull request overview

Adds optimistic concurrency to the MediatorFramework sample by introducing a version token (ROWVERSION in SQL / incrementing version in-memory), retrying transient concurrency conflicts via a request-handler decorator, and documenting the deferred ETag/If-Match follow-up in the task board.

Changes:

• Add Version concurrency token to GreetingResponse and persist it via SQL ROWVERSION + in-memory versioning.
• Add OptimisticConcurrencyRetrierDecorator<,> and register it for all IRequestHandler<,> to retry optimistic conflicts.
• Add initial tests for the retrier and in-memory stale-version rejection; update SMP-04 task docs as complete.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
samples/Ark.MediatorFramework.Sample/test/Ark.MediatorFramework.Sample.Tests/OptimisticConcurrencyTests.cs	Adds tests for retrier behavior and in-memory stale-version rejection.
samples/Ark.MediatorFramework.Sample/src/Ark.MediatorFramework.Sample.Database/dbo/Tables/Greeting.sql	Adds ROWVERSION column for SQL optimistic concurrency.
samples/Ark.MediatorFramework.Sample/src/Ark.MediatorFramework.Sample.Application/SampleDataContext.cs	Adds version-aware SQL persistence and reads/writes the Version token.
samples/Ark.MediatorFramework.Sample/src/Ark.MediatorFramework.Sample.Application/OptimisticConcurrencyRetrierDecorator.cs	Introduces retry decorator for optimistic concurrency exceptions.
samples/Ark.MediatorFramework.Sample/src/Ark.MediatorFramework.Sample.Application/GreetingStore.cs	Adds in-memory versioning and stale-version detection.
samples/Ark.MediatorFramework.Sample/src/Ark.MediatorFramework.Sample.Application/GreetingContracts.cs	Extends response contract with Version token for future transport-level concurrency.
samples/Ark.MediatorFramework.Sample/src/Ark.MediatorFramework.Sample.Application/ApplicationComposition.cs	Registers the retry decorator for request handlers.
docs/mediator-framework/progress/tasks/sample-parity/SMP-04-optimistic-concurrency.md	Marks ETag demo as explicitly deferred due to missing generator support.
docs/mediator-framework/progress/tasks/README.md	Marks SMP-04 as completed.

Comments suppressed due to low confidence (1)

samples/Ark.MediatorFramework.Sample/src/Ark.MediatorFramework.Sample.Application/SampleDataContext.cs:88

• SaveAsync falls back to the MERGE upsert path when greeting.Version is null, which means an existing greeting can be overwritten without any optimistic-concurrency check. This is inconsistent with InMemoryGreetingStore (which rejects updates without a version) and undermines the goal of version-aware persistence.

        const string sql = """
            MERGE [dbo].[Greeting] AS target
            USING (SELECT @Id AS [Id]) AS source ON target.[Id] = source.[Id]
            WHEN MATCHED THEN UPDATE SET [Message] = @Message, [Date] = @Date,
                [DateTime] = @DateTime, [OffsetDateTime] = @OffsetDateTime, [Period] = @Period,

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