.NET Workflows - Fix and re-enable `ValidateMultiTurnAsync` in Declarative Workflow integration tests

[crickman]

Additional: Hanging multi-turn declarative integration tests

The following integration tests in dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.IntegrationTests/DeclarativeWorkflowTest.cs hang indefinitely in CI (merge queue) and have been skipped with [Theory(Skip = "Multi-turn tests hang in CI - needs investigation")]:

• ValidateMultiTurnAsync("ConfirmInput.yaml", "ConfirmInput.json", false)
• ValidateMultiTurnAsync("RequestExternalInput.yaml", "RequestExternalInput.json", false)

Details

• The hang is SDK-independent - reproduces with both Azure.AI.Projects 1.2.0-beta.5 and 2.0.0-beta.1.
• The hang occurs in the workflow multi-turn execution path (useJsonCheckpoint: true).
• All other declarative integration tests (ValidateCaseAsync, ValidateScenarioAsync, FunctionCalling, InvokeTool, MediaInput) pass normally.
• The DeclarativeCodeGenTest.ValidateMultiTurnAsync was already skipped with Skip = "Needs template support".

Action needed

• Investigate the multi-turn workflow execution hang (likely related to checkpoint restore or the streaming run loop).
• Fix the tests and remove the Skip attribute.

Skipped in PR #4270.