fix: ensure streaming chunks are immediately flushed to console (#6424)

Added `flush=True` to the `aprint` call when handling
`ModelClientStreamingChunkEvent` message to ensure each chunk is
immediately displayed as it arrives.

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## Why are these changes needed?
When handling `ModelClientStreamingChunkEvent` message, streaming chunks
weren't guaranteed to be displayed immediately, as Python's stdout might
buffer output without an explicit flush instruction. This could cause
visual delays between when `chunk_event` objects are added to the
message queue and when users actually see the content rendered in the
console.
<!-- Please give a short summary of the change and the problem this
solves. -->

## Related issue number
None

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## Checks

- [x] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [x] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [x] I've made sure all auto checks have passed.

python/packages/autogen-agentchat/src/autogen_agentchat/ui/_console.py

@@ -177,7 +177,7 @@ async def Console(
                     f"{'-' * 10} {message.__class__.__name__} ({message.source}) {'-' * 10}", end="\n", flush=True
                 )
             if isinstance(message, ModelClientStreamingChunkEvent):
-                await aprint(message.to_text(), end="")
+                await aprint(message.to_text(), end="", flush=True)
                 streaming_chunks.append(message.content)
             else:
                 if streaming_chunks: