.NET: Fix broken workflow samples

[peibekwe]

Description

Fixing some workflow samples that were missed during polymorphic routing changes.

Fixes #4144

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

Updates .NET workflow samples to work with polymorphic routing changes by explicitly declaring message/output types for executors.

Changes:

• Add [SendsMessage] annotations to executors that dispatch messages so routing can resolve the intended message types.
• Add [YieldsOutput] annotations to executors that emit final workflow outputs.
• Minor formatting fix in the Loop sample (statement terminator).

Reviewed changes

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

Show a summary per file

File	Description
dotnet/samples/03-workflows/_StartHere/06_MixedWorkflowAgentsAndExecutors/Program.cs	Declares sent message types (ChatMessage, TurnToken) for executors involved in mixed agent/executor routing.
dotnet/samples/03-workflows/SharedStates/Program.cs	Declares the aggregation executor’s yielded output type.
dotnet/samples/03-workflows/Loop/Program.cs	Declares executor send/yield types and fixes a formatting issue in YieldOutputAsync call.
dotnet/samples/03-workflows/HumanInTheLoop/HumanInTheLoopBasic/WorkflowFactory.cs	Declares judge executor send/yield types for correct routing/output behavior.
dotnet/samples/03-workflows/ConditionalEdges/03_MultiSelection/Program.cs	Declares output yielding for terminal executors in multi-selection flow.
dotnet/samples/03-workflows/ConditionalEdges/02_SwitchCase/Program.cs	Declares output yielding for terminal executors in switch-case flow.
dotnet/samples/03-workflows/ConditionalEdges/01_EdgeCondition/Program.cs	Declares output yielding for terminal executors in edge-condition flow.
dotnet/samples/03-workflows/Concurrent/MapReduce/Program.cs	Declares sent message types for each stage and the final yielded output type for completion.
dotnet/samples/03-workflows/Concurrent/Concurrent/Program.cs	Declares sent message/output types for concurrent start/aggregation executors.
dotnet/samples/03-workflows/Checkpoint/CheckpointWithHumanInTheLoop/WorkflowFactory.cs	Declares judge executor send/yield types for checkpoint + HITL scenario.
dotnet/samples/03-workflows/Checkpoint/CheckpointAndResume/WorkflowFactory.cs	Declares guess/judge executor send/yield types for checkpoint-and-resume scenario.
dotnet/samples/03-workflows/Checkpoint/CheckpointAndRehydrate/WorkflowFactory.cs	Declares guess/judge executor send/yield types for checkpoint-and-rehydrate scenario.
dotnet/samples/03-workflows/Agents/WorkflowAsAnAgent/WorkflowFactory.cs	Declares yielded output type for workflow-as-agent aggregation executor.