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

[crickman]

Additional: Skipped CheckSystem declarative tests

The following integration tests have been skipped with [InlineData("CheckSystem.yaml", "CheckSystem.json", Skip = "Temporarily skipped")]:

• DeclarativeWorkflowTest.ValidateCaseAsync(workflowFileName: "CheckSystem.yaml", testcaseFileName: "CheckSystem.json", externalConveration: False)
• DeclarativeCodeGenTest.ValidateCaseAsync(workflowFileName: "CheckSystem.yaml", testcaseFileName: "CheckSystem.json", externalConveration: False)

Files modified

• dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.IntegrationTests/DeclarativeWorkflowTest.cs
• dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.IntegrationTests/DeclarativeCodeGenTest.cs

Action needed

• Investigate and fix the CheckSystem test failures.
• Remove the Skip attribute once fixed.

Skipped in PR #4270.