The Thinking disclosure rendered blank for every reasoning-emitting model
(Fable, DeepSeek, GPT-5.5, ...). Two causes:
1. ReasoningTextPart read a `text` prop that assistant-ui never populates —
reasoning parts arrive via context, same as text parts — so it always got
an empty string. Read the text via useMessagePartReasoning() instead,
mirroring how MarkdownText uses useMessagePartText().
2. The reasoning-only SmoothStreamingText / useSmoothReveal layer stalled at
revealed="": the reasoning part stays isRunning for the whole message while
the answer streams and thrashes re-renders, so the char-reveal never
advanced past 0. Render reasoning through the same DeferStreamingText →
surface path the assistant answer uses, and drop the dead smoothing code.
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