Bump third_party/openthread/repo from b200c96 to 9b99fbf (#27196)

Bumps [third_party/openthread/repo](/~https://github.com/openthread/openthread) from `b200c96` to `9b99fbf`.
- [Release notes](/~https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@b200c96...9b99fbf)

---
updated-dependencies:
- dependency-name: third_party/openthread/repo
  dependency-type: direct:production
...

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