fix(plugin): retry config readback during restart

[huanghuoguoguo]

Summary

• Retry Plugin config readback briefly when Core returns 404 while restarting the Plugin after a successful config write.
• Preserve the existing config equality check and fail immediately on other readback errors.

Validation

• make check build passed.
• A regression test covers one transient 404 followed by a successful readback.
• On an isolated LangBot instance, the previous CLI reported a readback failure although the config was saved. The patched CLI updated the QA RAG probe config with verified: true, then restored its original config with verified: true.