OnAfterMessageSentEvent not dispatched in streaming output path

[bingchengcc]

Bug Description

In RespondStage.process(), the streaming output path (ResultContentType.STREAMING_RESULT) calls send_streaming() and then early-returns without dispatching OnAfterMessageSentEvent. The non-streaming path correctly dispatches this event before clear_result().

This means plugins hooking into after_message_sent (e.g. simple_memory _capture) never fire for streaming platforms such as webchat.

Reproduction

1. Use a streaming-capable platform (e.g. webchat)
2. Add a plugin that hooks after_message_sent
3. Send a message — the hook never fires

Fix

Dispatch OnAfterMessageSentEvent and call clear_result() in the streaming path, matching the non-streaming path. Extract the shared logic into a _after_sent_cleanup helper to prevent the two paths from diverging again.

Fix in #9733