fix(agent): emit ThinkingBlockEnd before TextBlockStart at reasoning→answer boundary

[qbc2016]

AgentScope Version

2.0.2

Description

In _convert_chat_response_to_event, text blocks were processed before thinking blocks, causing ThinkingBlockEndEvent to be emitted after TextBlockDeltaEvent at the reasoning→answer boundary. This trapped the
first answer token(s) inside the <think> block for any downstream renderer.

Fix the ordering by splitting the logic into three explicit steps per chunk:

1. Close any open blocks absent from the current chunk (thinking first, then text)
2. Open/continue the thinking stream
3. Open/continue the text stream

This ensures ThinkingBlockEnd → TextBlockStart → TextBlockDelta at the boundary, and also fixes the edge case where a single chunk carries both a ThinkingBlock and a TextBlock.

Fixes #1885.

Checklist

Please check the following items before code is ready to be reviewed.

• An issue has been created for this PR
• I have read the CONTRIBUTING.md
• Docstrings are in Google style
• Related documentation has been updated (e.g. links, examples, etc.) in documentation repository
• Code is ready for review