Newer Claude Code CLIs emit stream message types the bundled SDK
was not compiled against — `rate_limit_event` being the one that
surfaced — and serde aborted the whole chat stream on the first
one ("unknown variant rate_limit_event").
The `Message` enum gains a `#[serde(other)] Unknown` catch-all so
any unmodelled `type` deserializes cleanly instead of failing the
parse; `hooks.rs` and `chat_claude.rs` match it as a silent no-op
(an unknown event carries no hook payload and no chat turn).
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