fix(reasoning): keep the measured duration in plain state

[RaphaelFakhri]

Fixes #496

Reasoning stored duration in useControllableState. A caller that passes duration={undefined} while streaming and a number once reasoning finishes switches the hook from uncontrolled to controlled. React logs this warning in development for every finished reasoning block:

undefined is changing from uncontrolled to controlled. Components should not switch from controlled to uncontrolled (or vice versa).

duration has no change callback, so the controllable-state hook adds nothing. This change keeps the measured duration in useState and uses durationProp ?? measuredDuration. The displayed value is the same in both modes: the prop when it is defined, otherwise the internal timer.

Tests

• does not warn when duration is set after streaming ends renders with duration={undefined} and isStreaming, then rerenders with duration={3}. It fails before the change and passes after.
• prefers the duration prop over the measured duration checks that a prop value wins over the timer.

cd packages/elements
npx vitest run __tests__/reasoning.test.tsx

Before the change: 2 failed, 18 passed. After: 20 passed. The full packages/elements suite passes (47 files, 960 tests).

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