Synced to upstream `vercel/chat@4.31.0`. The mapped-core **test** files (`packages/chat/src/*.test.ts`) are byte-identical between the `chat@4.30.0` and `chat@4.31.0` tags, so the fidelity re-pin to `chat@4.31.0` is string-only (732/732 mapped-core tests still pass, 0 missing); the core **source** delta is the `LinkButton` stable-id field (below). The headline is the **Linear agent-sessions** mode, plus the **Teams SDK-free primitive subpaths**, the **Slack 4.31** changes, **Telegram rich messages**, and a Python-only opt-in **`ThinkingChunk`** stream type. Sets `UPSTREAM_PARITY = "4.31.0"`.
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