Keep single-step details and focus through status updates

[Maya-Kid]

Why

When its only tool call finishes, StepGroup replaces the grouped StepRow with a new standalone row. This closes details the user is reading and removes the focused button, sending keyboard focus to the page body.

What changes

Keep a single call in the same StepRow throughout its status changes, keyed by the call's id so another call starts closed. A single running call now shows its own row instead of an extra one-step group; multiple calls keep their existing grouping.

Extend the existing tool-steps demo with timed success and failure controls so the transition can be inspected. No new public API, example name, or error/patch rendering behavior is introduced.

How it was checked

• On Linux with Node 24.13.0: fresh npm ci, npm run docs, npm run typecheck, and npm run build pass.
• Chromium 145 / Playwright 1.58.2: complete /dev/examples reports 389/389 mounted and 0 failed in both themes, without page errors.
• Tested the actual example's asynchronous success and failure while its row has keyboard focus; details remain open and the same button stays focused.
• Checked Enter/Space, unopened calls, initial completed calls, external focus, replacement ids, multiple-call grouping, and single/multiple boundaries without state leaking to another call.
• Visually inspected success and failure details in both themes. On the actual documentation page at a 1440px viewport, the expanded demo fits inside its existing 288px preview without clipping.

Before review

• Every commit is signed off (git commit -s)
• npm run typecheck and npm run build pass
• /dev/examples reports 0 failed
• What the change touches was looked at in both themes, open states included
• A component of Solid Glass's own has its example in src/examples/ and its page in content/solid/