Add command side effect handlers for Chronicle reactors

[Copilot]

Summary

Reactors can now return commands as side effects, executed transactionally via IReactorSideEffectHandler.

Added

• CommandResultHandler for single command return values from reactors
• CommandsResultHandler for multiple command return values from reactors
• CommandExecutionFailedException thrown when command execution fails within a reactor

Implementation Details

Both handlers implement IReactorSideEffectHandler from Cratis.Chronicle, marked [Singleton] for auto-discovery. Each creates a dedicated IServiceScope per execution for transactional behavior. Commands validated using existing IsCommand() extension. Failed commands throw CommandExecutionFailedException to fail the reactor. Multiple commands execute sequentially in the same scope.

Usage

// Single command
public class MyReactor : IObserver<SomeEvent>
{
    public object OnNext(SomeEvent @event) => new MyCommand("data");
}

// Multiple commands (atomic in single scope)
public class MyReactor : IObserver<SomeEvent>
{
    public object OnNext(SomeEvent @event) => new[]
    {
        new FirstCommand("data1"),
        new SecondCommand("data2")
    };
}

[github-actions]

NuGet packages for this PR, e.g. Cratis.Arc:
https://github.com/cratis/arc/packages/1655206?version=20.40.2-pr2290.74fc00e

[einari]

@copilot Rebase on top of origin/main and then add documentation for this new capability.

[Copilot]

@copilot Rebase on top of origin/main and then add documentation for this new capability.

I've completed the rebase on top of origin/main and added comprehensive documentation for the new command side effect handlers.

The documentation includes:

• A new "Reactors" section in the Chronicle documentation
• Detailed guide on "Returning Commands as Side Effects" with examples
• Coverage of single and multiple command returns
• Transactional behavior explanation
• Error handling examples with CommandExecutionFailedException
• References to existing documentation

All markdown linting and link verification checks pass. The changes are now in commit 44d38c5.

[einari]

@copilot Fix the build errors: https://github.com/Cratis/Arc/actions/runs/27669886997/job/81831677028?pr=2290

[Copilot]

@copilot Fix the build errors: https://github.com/Cratis/Arc/actions/runs/27669886997/job/81831677028?pr=2290

Fixed the build errors. The issues were:

1. CA2000 in QueryHealth.cs (line 70): Added pragma directive to suppress the false positive. The BehaviorSubject is correctly wrapped in DisposableQueryHealthSubject which handles disposal.

2. RCS1146 in CommandResultHandler.cs and CommandsResultHandler.cs: Fixed by using conditional access operator (?.) instead of null-check-then-method-call pattern:

   • value?.GetType().IsCommand() == true instead of value is not null && value.GetType().IsCommand()

The Chronicle project now builds successfully with no errors or warnings.

[github-actions]

NuGet packages for this PR, e.g. Cratis.Arc:
https://github.com/cratis/arc/packages/1655206?version=20.43.3