[codex] Fix s16 protocol flowchart labels

[chaochaoweb3]

Summary

• make the main request-response row explicitly describe the shutdown flow
• show submit_plan -> review_plan -> plan_approval_response as the separate plan approval flow
• clarify that request_plan is a plain message, not a protocol request

Fixes #373.

Validation

• parsed all three updated SVG files with xml.etree.ElementTree
• git diff --check

[vercel]

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[Bill-Billion]

之前的图片并没有问题，修改后的图片则出现了字的错位，请确保是课程中存在的问题，再去深入研究提出PR。