[Bug]: ACP delegate_external_agent sometimes returns "completed without text output" when notifications race the prompt response

## Description

When an external ACP agent writes a `session/update` notification and the `session/prompt` response back-to-back (e.g. same TCP segment), `delegate_external_agent` sometimes returns **"completed without text output"** even though the agent produced text.

## Root cause

The ACP transport (external `acp` SDK) resolves the prompt response future **inline**, while the notification runner is still queued as a **separate task**. In `ACPService._wait_for_prompt_outcome()`, `finish_prompt()` is called immediately after the prompt future resolves — at that point the notification carrying the final text may not have been processed yet, so `_assistant_text` is empty and the final text is dropped.

## Repro

Occurs reliably when the agent server flushes the notification and the response together in one TCP segment / readline batch. The server-side symptom matches the race documented in the ACP SDK transport.

## Fix (PR #6623)

Two complementary changes:
1. `client.py`: `session_update()` emits accumulated text immediately on `AgentMessageChunk` (in addition to the `finish_prompt()` flush), so streaming consumers capture the text via `on_message`.
2. `service.py`: `_wait_for_prompt_outcome()` yields to the event loop (`await asyncio.sleep(0)`) before calling `finish_prompt()`.

See https://github.com/agentscope-ai/QwenPaw/pull/6623

## Environment

- QwenPaw version: main (2026-08-01)
- OS: Linux (aarch64)