test: Unit tests for IMediatorSendOnly

[Copilot]

Adds contract and DI integration tests for IMediatorSendOnly to guard against regressions in the interface hierarchy and service registration.

Changes

• New file: IMediatorSendOnlyTests.cs — four focused tests:
  • IMediator is assignable to IMediatorSendOnly (inheritance contract)
  • IMediatorSendOnly declares neither QueryAsync nor StreamQueryAsync (write-only boundary enforcement via reflection)
  • Resolving IMediatorSendOnly from DI returns the same scoped instance as IMediator
  • A consumer typed to IMediatorSendOnly can invoke both SendAsync and PublishAsync

// Write-only consumer — compile-time CQRS enforcement
private sealed class WriteOnlyConsumer
{
    private readonly IMediatorSendOnly _mediator;

    public WriteOnlyConsumer(IMediatorSendOnly mediator) => _mediator = mediator;

    public async Task ExecuteAsync(CancellationToken cancellationToken = default)
    {
        _ = await _mediator.SendAsync<TestCommand, string>(new TestCommand(), cancellationToken);
        await _mediator.PublishAsync(new TestEvent(), cancellationToken);
    }
}

No production code changes required.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.19%. Comparing base (380596b) to head (7cbc378).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #403      +/-   ##
==========================================
+ Coverage   91.98%   92.19%   +0.20%     
==========================================
  Files         139      139              
  Lines        5266     5266              
  Branches      484      484              
==========================================
+ Hits         4844     4855      +11     
+ Misses        282      272      -10     
+ Partials      140      139       -1     

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