Add builder-style API for ForkJoin tasks with fluent subtask configuration

[Copilot]

This PR addresses the feedback from PR #1 by implementing a builder-style approach for configuring ForkJoin tasks, replacing the cumbersome object-based configuration with a fluent API similar to the existing DoWhile task builder.

Problem

The current ForkJoin implementation requires manually creating arrays of WorkflowTask objects, which is verbose and error-prone:

_builder.AddTask(
    wf => wf.ForkJoinTask,
    wf => new ForkJoinInput()
    {
        ForkTasks = new[]
        {
            new WorkflowTask[]
            {
                new()
                {
                    Name = "get_customer_task",
                    TaskReferenceName = "get_customer_ref",
                    Type = "SIMPLE",
                    WorkflowTaskType = WorkflowTaskType.SIMPLE,
                    InputParameters = new Dictionary<string, object> { { "customerId", wf.WorkflowInput.CustomerId } }
                }
            },
            // More manual WorkflowTask creation...
        },
        JoinOn = new[] { "get_customer_ref", "prepare_email_ref" }
    }
);

Solution

The new implementation provides a clean, fluent API that follows the established pattern from DoWhile tasks:

_builder.Fork(
    wf => wf.ForkJoinTask,
    forkBuilder =>
    {
        // First parallel branch
        forkBuilder.AddFork(taskBuilder =>
        {
            taskBuilder.AddTask(
                wf => wf.GetCustomer,
                wf => new CustomerGetV1Input { CustomerId = wf.WorkflowInput.CustomerId }
            );
        });

        // Second parallel branch  
        forkBuilder.AddFork(taskBuilder =>
        {
            taskBuilder.AddTask(
                wf => wf.PrepareEmail,
                wf => new EmailPrepareV1Input { Name = "Default", Address = "Default" }
            );
        });
    }
);

Implementation Details

• IForkBuilder<TWorkflow>: Interface for configuring fork branches
• ForkTaskBuilder<TWorkflow>: Main builder that extends BaseTaskBuilder and manages multiple parallel branches
• ForkBranchBuilder<TWorkflow>: Individual branch builder implementing ITaskSequenceBuilder for task sequences
• Fork() extension method: Entry point providing the fluent API

Key Features

• Multiple tasks per branch: Each fork branch can contain sequential tasks
• Automatic task reference collection: JoinOn property is automatically populated from all branch tasks
• Full backward compatibility: Existing object-based approach continues to work
• Consistent with existing patterns: Follows the same builder pattern as DoWhile tasks
• Type safety: Leverages existing strongly-typed task models

Generated Output

The builder generates the same FORK_JOIN and JOIN workflow structure as the original implementation, ensuring compatibility with existing Conductor workflows.

Examples Added

• ForkBuilderWorkflow: Basic demonstration of the new API
• ForkJoinWorkflowNewStyle: Migration example showing old vs new approach
• MultiBranchForkWorkflow: Advanced example with multiple tasks per branch

This enhancement significantly improves the developer experience when creating parallel workflows while maintaining full compatibility with existing code.

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