Adds a Context Overflow Errors subsection to custom-provider.md showing how a custom provider extension can rewrite unrecognized overflow errors via a message_end handler so pi's native compact-and-retry path triggers. Includes guard rails for provider scoping, idempotency, and avoiding rate-limit phrases.
The notify() API only accepts "info" | "warning" | "error" — "success"
was a copy-paste error in the example.
Co-Authored-By: julien-agent <Agents+cyolo@huggingface.co>