feat: WorkflowHostAgent forwards AgentResponseEvent unconditionally under Futures-on

Aligns the .NET Workflow-as-Agent surface with Python `as_agent`. Under
`Futures.EnableAgentResponseOutputTaggingAndFiltering = true`,
`WorkflowSession.InvokeStageAsync` now forwards `AgentResponseEvent`
unconditionally — joining `AgentResponseUpdateEvent` in ignoring the host's
`includeWorkflowOutputsInResponse` switch. That switch keeps governing the
generic `WorkflowOutputEvent` path for non-AIAgent payloads, where it is
further short-circuited by an `IsIntermediate()` check (tagged intermediate
outputs always surface).

Under Futures-off the legacy asymmetry is preserved: `AgentResponseUpdateEvent`
always forwarded, `AgentResponseEvent` gated by `includeWorkflowOutputsInResponse`.

Back-compat: with `Futures.EnableAgentResponseOutputTaggingAndFiltering` left at
its default `false`, observable behavior is identical to before.

`Futures` documentation gains a remark explaining the `Workflow.AsAIAgent()`
interaction in both flag states.

Runner fix
----------
`InProcessRunnerContext.YieldOutputAsync` now skips `Executor.CanOutput` for
AgentResponse-shaped payloads under both Futures branches. `AIAgentHostExecutor`
doesn't declare AgentResponse(Update) in its `Yields` set, so the historical
legacy bypass had silently skipped the check; Phase 3's Futures-on path was
running it and would reject AIAgent payloads. AIAgent-shaped payloads are now
always a valid output shape, matching the legacy bypass semantics.

Phase 4 follow-on
-----------------
Switched the three orchestration-builder designation-replay loops to iterate
`Dictionary.Keys` with a value lookup instead of constructing/destructuring
`KeyValuePair<,>`. Cleaner shape and avoids the netstandard2.0 / net472
`KeyValuePair<,>.Deconstruct` unavailability that surfaced when this branch
multi-TFM-built.

Tests
-----
`WorkflowHostSmokeTests.IntermediateForwarding` (new nested class, 6 tests):
- intermediate AgentResponse forwarded past the include-outputs gate (Futures on)
- terminal AgentResponse forwarded unconditionally (Futures on)
- terminal AgentResponse gated by include flag (Futures off, legacy)
- undesignated AIAgent executor emits no AgentResponseEvent under Futures-on
- legacy bypass still emits AgentResponseEvent under Futures-off
- intermediate tag is observable via `update.RawRepresentation`

The class joins the `FuturesSerial` xUnit collection so the process-global flag
is serialized against other Futures-toggling tests.

599/599 unit tests pass on net10.0 (593 baseline + 6 new).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowSession.cs

@@ -520,7 +520,12 @@ internal sealed class WorkflowSession : AgentSession
                     goto default;
 
                 case AgentResponseEvent agentResponse:
-                    if (!this._includeWorkflowOutputsInResponse)
+                    // Under Futures.EnableAgentResponseOutputTaggingAndFiltering=true, mirror
+                    // AgentResponseUpdateEvent's behavior: always forward, regardless of the
+                    // _includeWorkflowOutputsInResponse host flag. Under the legacy default,
+                    // keep today's behavior — gated by the include flag.
+                    if (!Futures.EnableAgentResponseOutputTaggingAndFiltering &&
+                        !this._includeWorkflowOutputsInResponse)
                     {
                         goto default;
                     }
@@ -539,7 +544,11 @@ internal sealed class WorkflowSession : AgentSession
                         _ => null
                     };
 
-                    if (!this._includeWorkflowOutputsInResponse || updateMessages == null)
+                    // Same gating asymmetry as AgentResponseEvent: intermediate outputs are
+                    // forwarded unconditionally; terminal/untagged outputs require the host
+                    // to opt in via _includeWorkflowOutputsInResponse.
+                    if (updateMessages == null ||
+                        (!output.IsIntermediate() && !this._includeWorkflowOutputsInResponse))
                     {
                         goto default;
                     }