.NET: [Bug]: Declarative workflows silently skip unsupported actions

### Description

`WorkflowActionVisitor` routes 32 declarative action kinds to `NotSupported`. When a workflow contains one, the action is skipped, the run continues, and it completes successfully. Nothing reaches the caller to say that part of the workflow did not execute.

A workflow whose YAML calls `SearchKnowledgeSources` or `InvokeFlowAction` therefore reports success while never performing the retrieval or invoking the flow.

#### What happens

`NotSupported` is the whole of the handling:

```csharp
private void NotSupported(DialogAction item)
{
Debug.WriteLine($"> UNKNOWN: ...{FormatItem(item)} => {FormatParent(item)}");
this.HasUnsupportedActions = true;
}
```

Neither of those two signals is available to an application:

1. `Debug.WriteLine` is `System.Diagnostics.Debug`, which is `[Conditional("DEBUG")]`. The call is removed by the compiler in Release, so a shipped app gets no output at all.
2. `HasUnsupportedActions` is declared on `WorkflowActionVisitor`, which is `internal sealed`. It is not reachable from outside the assembly. Across the repository it is read in exactly one place, and that place is a unit test (`DeclarativeWorkflowTest`).

So in a Release build the skip leaves no trace anywhere.

#### Why this is awkward today

The visitor already has an `ILoggerFactory` on the same code path, and already uses it when it attaches a logger to each executor:

```csharp
executor.Logger = this._workflowOptions.LoggerFactory.CreateLogger(executor.Id);
```

A warning through that factory, naming the action kind and id, would make the skip visible without changing any control flow.

#### The actions currently affected

`ActivateExternalTrigger`, `AdaptiveCardPrompt`, `AnswerQuestionWithAI`, `BeginDialog`, `CSATQuestion`, `CreateSearchQuery`, `DeleteActivity`, `DisableTrigger`, `DisconnectedNodeContainer`, `EmitEvent`, `GetActivityMembers`, `GetConversationMembers`, `InvokeAIBuilderModelAction`, `InvokeConnectorAction`, `InvokeCustomModelAction`, `InvokeFlowAction`, `InvokeSkillAction`, `LogCustomTelemetryEvent`, `OAuthInput`, `RecognizeIntent`, `RepeatDialog`, `ReplaceDialog`, `SearchAndSummarizeContent`, `SearchAndSummarizeWithCustomModel`, `SearchKnowledgeSources`, `SignOutUser`, `TransferConversation`, `TransferConversationV2`, `UnknownDialogAction`, `UpdateActivity`, `WaitForConnectorTrigger`.

I appreciate this list is expected to shrink as more actions gain support, which is what `HasUnsupportedActions` looks like it was put there to track. The gap is that nothing surfaces it while an action is still on the list.

### Error Messages / Stack Traces

None, and that is the substance of the report. The workflow completes without an error, a warning, or a log line.

### Package Versions

Microsoft.Agents.AI.Workflows.Declarative

### .NET Version

.NET 10.0

### Additional Context

Verified on `main` at commit 4507512f.

If you agree the silent skip is unintended, I would like to take this on. My thinking is a warning logged through the existing `LoggerFactory` from inside `NotSupported`, plus a unit test, since that changes no control flow and adds no public API surface. Happy to go a different way if you would rather surface it through the workflow model or an event, or to close this if the current behavior is deliberate.