Add ForkJoin task support for simplified parallel execution

[Copilot]

This PR adds support for the ForkJoin operator in conductor-sharp, providing a simpler alternative to the existing DynamicForkJoin for cases where you just want to run tasks in parallel for performance reasons.

What's Added

ForkJoin Task Implementation

• ForkJoinTaskModel and ForkJoinInput models for type-safe workflow definition
• ForkJoinTaskBuilder that generates proper FORK_JOIN and JOIN tasks
• Extension method AddTask() for fluent workflow building

Key Features

• Static fork definition (vs dynamic at runtime)
• Simple parallel execution of predefined task branches
• Proper mapping of forkTasks and joinOn properties
• Full integration with existing workflow builder patterns

Usage Example

public class ParallelWorkflow : Workflow<ParallelWorkflow, WorkflowInput, WorkflowOutput>
{
    public ForkJoinTaskModel ParallelTasks { get; set; }

    public override void BuildDefinition()
    {
        _builder.AddTask(wf => wf.ParallelTasks, wf => new ForkJoinInput()
        {
            ForkTasks = new[]
            {
                new WorkflowTask[]
                {
                    new()
                    {
                        Name = "get_customer_task",
                        TaskReferenceName = "get_customer",
                        Type = "SIMPLE",
                        WorkflowTaskType = WorkflowTaskType.SIMPLE,
                        InputParameters = new Dictionary<string, object> { { "customerId", wf.WorkflowInput.CustomerId } }
                    }
                },
                new WorkflowTask[]
                {
                    new()
                    {
                        Name = "prepare_email_task",
                        TaskReferenceName = "prepare_email",
                        Type = "SIMPLE",
                        WorkflowTaskType = WorkflowTaskType.SIMPLE,
                        InputParameters = new Dictionary<string, object> { { "templateId", "welcome" } }
                    }
                }
            },
            JoinOn = new[] { "get_customer", "prepare_email" }
        });
    }
}

Generated Workflow Structure

The ForkJoin implementation generates two tasks:

1. FORK Task: Contains the parallel task branches in forkTasks property
2. JOIN Task: Waits for completion of specified tasks in joinOn property

This follows the standard Conductor ForkJoin pattern as documented at https://docs.conductor-oss.org/documentation/configuration/workflowdef/operators/fork-task.html

Testing

• Complete test workflow demonstrating usage
• Integration test verifying correct JSON workflow definition generation
• All existing tests continue to pass (33/33 passing)

This implementation provides a much simpler API compared to DynamicForkJoin when you have a known set of tasks to run in parallel, addressing the performance use case mentioned in the issue.

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[Roruchi]

Can you make sure it does not use objects for the fork tasks, but instead i can add it through builder style. Like in the do-while task builder i.e. this is some code on how we can potentially use it:

builder.Fork(
            wf => wf.ForkReference
            forkBuilder =>
            {
forkBuilder.AddFork(taskBuilder => {
  taskBuilder.AddTask(
                    wf => wf.Task1,
                    wf => new Task!Input
                    {
                        Name = wf.Task0.Ouput
                    }
                );
                taskBuilder.AddTask(
                    wf => wf.Task2,
                    wf => new Task2Input
                    {
                         Name = wf.Task1.Ouput
                    }
                );
            }
} 
        );
        

[Roruchi]

Can you make sure it does not use objects for the fork tasks, but instead i can add it through builder style. Like in the do-while task builder i.e. this is some code on how we can potentially use it:

builder.Fork(
wf => wf.ForkReference
forkBuilder =>
{
forkBuilder.AddFork(taskBuilder => {
taskBuilder.AddTask(
wf => wf.Task1,
wf => new Task!Input
{
Name = wf.Task0.Ouput
}
);
taskBuilder.AddTask(
wf => wf.Task2,
wf => new Task2Input
{
Name = wf.Task1.Ouput
}
);
}
}
);