.NET: [Feature Branch] Add nested sub-workflow support for durable workflows

[kshyju]

Summary

Adds support for nested sub-workflows in the Durable Task workflow system. Sub-workflows can now act as executors within other workflows, including multi-level nesting (sub-workflow within sub-workflow).

Changes

Library (Microsoft.Agents.AI.DurableTask)

• DurableExecutorDispatcher — Added sub-workflow dispatch via CallSubOrchestratorAsync, with conversion of DurableWorkflowResult to DurableExecutorOutput for seamless integration with the parent workflow's result processing.
• DurableWorkflowResult — Added SentMessages property (List<TypedPayload>) so sub-workflow results carry typed messages that get routed to connected executors in the parent workflow.
• DurableWorkflowRunner — The final workflow result is now included as a SentMessage so parent workflows can route it to connected executors via the edge map.

Public API Changes

None.

Sample (07_SubWorkflows)

New console app sample demonstrating:

• Sub-workflows: Using Workflow.BindAsExecutor() to embed a workflow as an executor
• Multi-level nesting: FraudCheck sub-workflow nested inside a Payment sub-workflow (Level 2)
• Automatic discovery: Only the top-level workflow is registered; sub-workflows are discovered automatically
• Event propagation: Custom FraudRiskAssessedEvent bubbles up from nested sub-workflows
• Message passing: SelectCarrier uses Executor<TInput> (void return) with SendMessageAsync to forward typed messages to connected executors
• Shared state: AnalyzePatterns → CalculateRiskScore communicate via QueueStateUpdateAsync/ReadStateAsync

OrderProcessing (main workflow)
├── OrderReceived
├── Payment (sub-workflow)
│   ├── ValidatePayment
│   ├── FraudCheck (sub-sub-workflow)
│   │   ├── AnalyzePatterns
│   │   └── CalculateRiskScore
│   └── ChargePayment
├── Shipping (sub-workflow)
│   ├── SelectCarrier (void-return + SendMessageAsync)
│   └── CreateShipment
└── OrderCompleted

Tests

• Added SubWorkflowsSampleValidationAsync integration test to WorkflowConsoleAppSamplesValidation — validates all executor log entries across all three nesting levels, verifies event propagation, and confirms end-to-end workflow completion.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This pull request adds support for nested sub-workflows in the Durable Task workflow system, enabling workflows to act as executors within other workflows with multi-level nesting capabilities (sub-workflow within sub-workflow). The implementation extends the existing durable workflow infrastructure to support hierarchical workflow execution via Durable Task sub-orchestrations, providing independent checkpointing, replay protection, and dashboard visualization for each sub-workflow instance.

Changes:

• Extended DurableWorkflowResult with a SentMessages property to enable message routing from sub-workflows to parent workflow executors
• Enhanced DurableExecutorDispatcher to detect and dispatch sub-workflow executors via CallSubOrchestratorAsync with automatic result format conversion
• Modified DurableWorkflowRunner to include the final workflow result as a sent message for proper routing in parent workflows
• Added comprehensive sample (07_SubWorkflows) demonstrating 2-level workflow nesting, event propagation, and shared state within sub-workflows
• Added integration test validating all executor execution across three nesting levels and event propagation

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/DurableWorkflowResult.cs	Added SentMessages property to carry typed messages from sub-workflows to parent workflows
dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/DurableExecutorDispatcher.cs	Added ExecuteSubWorkflowAsync method to dispatch sub-workflows as sub-orchestrations and convert results to executor output format
dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/DurableWorkflowRunner.cs	Modified to include final result as a sent message and added null-conditional operators for defensive deserialization
dotnet/tests/Microsoft.Agents.AI.DurableTask.IntegrationTests/WorkflowConsoleAppSamplesValidation.cs	Added SubWorkflowsSampleValidationAsync test validating all executors across three nesting levels
dotnet/samples/Durable/Workflow/ConsoleApps/07_SubWorkflows/Program.cs	Sample entry point demonstrating 2-level nested sub-workflow configuration and execution
dotnet/samples/Durable/Workflow/ConsoleApps/07_SubWorkflows/Executors.cs	Executor implementations showcasing sub-workflow features including event propagation and shared state
dotnet/samples/Durable/Workflow/ConsoleApps/07_SubWorkflows/README.md	Comprehensive documentation explaining sub-workflow concepts and sample structure
dotnet/samples/Durable/Workflow/ConsoleApps/07_SubWorkflows/07_SubWorkflows.csproj	Project configuration for the sub-workflows sample
dotnet/agent-framework-dotnet.slnx	Added sample project to solution file

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

[cgillum]

Great to see this feature landing. Just a few comments.

[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 5 comments.

[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated no new comments.

[cgillum]

Looks good - just one minor comment.