feat(chat): add fallbacks for unhandled tools

[FabienMotte]

Summary

This PR adds fallbacks for unhandled tools:

• In development, it throws an error asking the user to provide a tool implementation using the tools prop.
• In production, it adds a tool result with the output "No tool implemented for ...", which triggers a new request to the agent.

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Sandbox	Source
example-react-instantsearch-getting-started	Configuration
example-react-instantsearch-next-app-dir-example	Configuration
example-react-instantsearch-next-routing-example	Configuration
example-vue-instantsearch-getting-started	Configuration

[Haroenv]

looks like some tests should be updated

[Haroenv]

perfect, I think we can just move the "null" branch from within tools to outside