fix(react-ui): don't auto-open the last Accordion item on initial mount

[Shinyaigeek]

Summary

Fixes #861 — <Accordion> opens the last item on initial mount for a fully static (non-streaming) response, instead of the first.

The auto-open branch treats any growth in item count as "a new item arrived during streaming". On the baseline render prevItemCount is 0, so a static N-item response satisfies items.length > prevItemCount.current and opens items[items.length - 1]. This is the same defect shape as the <Tabs> issue #819: a streaming follow-along heuristic that doesn't exclude the initial mount.

Changes

• Baseline pass opens the first item. prevItemCount now starts at null so the very first render is distinguishable from streaming growth. A static mount opens the first section — restoring the behavior from before the refactor(react-ui): simplify Accordion auto-open logic #485 refactor removed the first-item mount effect. Streaming follow-along is unchanged: an item arriving after the baseline still auto-opens the newest one (an accordion that mounts empty mid-stream records a 0 baseline, so its first item still auto-opens on arrival).
• Track the auto-opened item by index, not by value. Previously, when a new item arrived before its value string had finished streaming, setOpenItem latched a prefix of the value (e.g. "ov" for "overview"); since the only re-sync trigger was count growth, the completed value never re-synced and every section ended up closed. Tracking the index re-syncs openItem once the value completes.

Reproduction / verification

Standalone repro against the published packages: https://github.com/Shinyaigeek/openui-accordion-repro

Before (published react-ui@0.13.1)	After (this branch, local build)

• Added an SSR regression test (Accordion.ssr.test.tsx) that renders the static three-section program through the real Renderer + openuiLibrary and asserts the first section is the open one. It fails on the previous implementation with [ 'false', 'false', 'true' ] (last open) and passes with this change.
• pnpm test in packages/react-ui: 28 passed.
• Verified in a browser via the repro with a local build of this branch: the first section (Apple) is open on mount.

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