Python: [Bug]: WorkflowAgent drops response metadata when streaming an AgentResponse output

@eavanvalkenburg is already working on this.

Since Sep 17, 2026.

### Description

When a workflow executor emits a message-bearing `AgentResponse` and the workflow is consumed through `Workflow.as_agent().run(stream=True)`, `WorkflowAgent` converts the response messages into `AgentResponseUpdate` objects but silently drops response-level metadata that the update type can represent:

- `agent_id`
- `finish_reason`
- `continuation_token`
- `additional_properties`

Actual behavior: the message content is preserved, but all four fields are `None` on the generated update.

Expected behavior: converting a message-bearing `AgentResponse` to streaming updates should preserve representable response-level metadata without changing message count or ordering.

### Code Sample

```python
import asyncio
from typing import Never

from agent_framework import (
AgentResponse,
Message,
WorkflowBuilder,
WorkflowContext,
executor,
)

@executor
async def emit_response(
_messages: list[Message],
ctx: WorkflowContext[Never, AgentResponse],
) -> None: # type: ignore[valid-type]
await ctx.yield_output(
AgentResponse(
messages=[Message(role="assistant", contents=["payload"])],
agent_id="source-agent",
finish_reason="length",
continuation_token={"token": "resume-token"},
additional_properties={"provider_marker": "preserve-me"},
)
)

async def main() -> None:
workflow = WorkflowBuilder(start_executor=emit_response).build()
updates = [
update
async for update in workflow.as_agent("wrapped-workflow").run(
"hello",
stream=True,
)
]

update = updates[0]
print(
{
"agent_id": update.agent_id,
"finish_reason": update.finish_reason,
"continuation_token": update.continuation_token,
"additional_properties": update.additional_properties,
}
)

asyncio.run(main())
```

### Error Messages / Stack Traces

No exception is raised.

On current `main`, the output is:

```text
{'agent_id': None, 'finish_reason': None, 'continuation_token': None, 'additional_properties': None}
```

### Package Versions

`agent-framework-core`: local source from `main` at `0275ebfd026e3549b9377788aedb58f424997fb2`

### Python Version

Python 3.11.14

### Additional Context

[#7952](https://github.com/microsoft/agent-framework/issues/7952) reported the equivalent metadata loss for the neighboring `AgentResponseUpdate` reconstruction branch. [#7999](https://github.com/microsoft/agent-framework/pull/7999) fixed that branch by preserving these fields and explicitly identified the `AgentResponse` branch as possible follow-up scope.

Existing workflow-agent tests cover streaming message content from an `AgentResponse`, but do not cover response metadata preservation on that conversion path.