.NET: Add unit tests for RequestExternalInputExecutor

[Copilot]

Motivation and Context

Unit test coverage was missing for RequestExternalInputExecutor. The executor has two public methods (ExecuteAsync and CaptureResponseAsync) that required different testing approaches due to extension method usage.

Description

Added 7 unit tests achieving 100% line and branch coverage:

ExecuteAsync tests (4 tests) - Standard workflow execution pattern:

• Naming convention validation for Steps.Input() and Steps.Capture()
• Execution with/without variable assignment
• Verification of IsDiscreteAction=false and EmitResultEvent=false

CaptureResponseAsync tests (3 tests) - Direct method invocation pattern:

• Capturing response without workflow conversation (variable assignment only)
• Capturing single message with conversation (calls WorkflowAgentProvider.CreateMessageAsync)
• Capturing multiple messages with conversation (iterates all messages)

Key technique: Extension methods (SetLastMessageAsync, GetWorkflowConversation, QueueStateUpdateAsync) work naturally by wrapping mock IWorkflowContext with WorkflowFormulaState:

Mock<IWorkflowContext> mockBaseContext = new(MockBehavior.Loose);
DeclarativeWorkflowContext context = new(mockBaseContext.Object, this.State);
await action.CaptureResponseAsync(context, response, CancellationToken.None);

This eliminates complex mocking of extension methods - they use the framework's existing implementation.

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.

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