The share-time validation forced every agent bound to a knowledge base to
have a rerank model configured, which blocked Wiki-type agents
(`wiki_search` / `wiki_read_page` / …) from being shared even though they
never invoke the reranker at runtime.
Align the check with the runtime logic in `session_agent_qa.go`: only
require a rerank model when the agent's allowed_tools include
`knowledge_search` (or rely on the default tool set, which contains it).
Wiki-only agents can now be shared without a rerank model; hybrid / RAG
agents still enforce the requirement.
Also update the handler error message to reflect the new rule.
wizardchen
b8c45b173e