.NET Workflows - Add unit tests for QuestionExecutor

[Copilot]

Motivation and Context

QuestionExecutor lacked unit test coverage. This PR adds comprehensive tests following existing patterns from ForeachExecutorTest to validate all code paths and handlers.

Description

Created QuestionExecutorTest with 15 tests covering:

Handler Methods

• ExecuteAsync - AlwaysPrompt, SkipQuestionMode variants, variable state conditions
• PrepareResponseAsync - prompt preparation and count increment
• CaptureResponseAsync - valid/invalid/unrecognized entity extraction, autoSend behavior, repeat count exceeded
• CompleteAsync - completion event emission

Static Methods & Utilities

• IsComplete - result state validation
• Steps naming convention (Prepare, Input, Capture)

Test Design

• Reusable CreateModel helper for Question model instantiation
• Distinct private methods per handler (ExecuteTestAsync, PrepareResponseTestAsync, CaptureResponseTestAsync, CompleteTestAsync)
• Mock WorkflowAgentProvider with Moq for isolated testing
• No var keyword, no underscores in test names, minimal namespace references

Coverage: 86.66% line coverage, all 15 tests passing

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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