docs: updates for v0.21.0 release - #4381

Would you be open to a small community-tools note in this testing guide?

I maintain AgentRunProof, a CLI that complements the provider-neutral scripted fixtures here by running the real Runner through deterministic cross-version RunState and approval-resume conformance scenarios. It emits content-addressed verification records and its built-in scenarios make no provider request and require no API key.

A deliberately small addition could be:

## Ecosystem tools

The following community tools complement the SDK's provider-neutral testing APIs.

- [AgentRunProof](https://github.com/FU-max-boop/agentrunproof) runs deterministic cross-version conformance scenarios for serialized `RunState` and approval-resume workflows, and emits reproducible certificates without provider requests.

This is the harness I used to diagnose the checkpoint-isolation gap that led to #4413 and to validate the recursive/serialized fix in #4414. It would add no SDK dependency or runtime behavior, and I would be happy to submit the one-file docs change after this release-docs PR lands if you prefer to keep release preparation separate.

The current PyPI release targets 0.20.x; I am preparing the explicit 0.21 compatibility release in parallel and will not claim 0.21 support until #4387 is published.

@FU-max-boop Thanks for sharing this, and for using your tool to help diagnose #4413 and validate #4414. We really appreciate contributions that use tools like this to find gaps and provide reproducible verification.

For now, we would like to keep this testing guide focused on SDK-maintained testing APIs, so we are not planning to add community tool listings here. That said, contributions and feedback backed by your tool's findings are very welcome.

Understood — thank you for the clear boundary and the kind words. I’ll keep AgentRunProof external to the SDK and use it to provide narrow, conventional regressions and reproducible before/after validation when it finds a concrete gap. I won’t pursue a community-tools listing in this guide.