test: lock single context-engine registration

[haosenwang1018]

Summary

• add a focused test that locks the OpenViking plugin to a single registerContextEngine(...) call
• protect the recent hook/context-engine coordination fixes from future duplicate-registration regressions
• keep the registration contract explicit in the existing tools/plugin registration test file

Testing

• git diff --check

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[Mijamind719]

您好，before_prompt_build：负责长期记忆召回
assemble()：负责上下文组装/压缩，这两个hook的定位就是要同时做，这个pr是想把before_prompt_build disable掉么？