fix(core/ui): guard against string-only messages in the error pill

[Copilot]

Clicking the error/warning pill to view details crashed when a message was a plain string instead of an object, since string messages produce a DocumentFragment with only text nodes (firstElementChild/lastElementChild are both null).

Root cause

• createWarnButton in src/core/ui.js unconditionally accessed fragment.firstElementChild.childNodes when firstElementChild === lastElementChild, which is also true (null === null) for text-only fragments, causing a TypeError on click.

Fix

• Added a null guard so the "single element" shortcut only applies when firstElementChild actually exists; string messages now fall through to the existing "take everything" branch, appending the raw text fragment to the list item.

if (
  fragment.firstElementChild &&
  fragment.firstElementChild === fragment.lastElementChild
) {
  li.append(...fragment.firstElementChild.childNodes);
} else {
  li.appendChild(fragment);
}

Tests

• Added a regression test in tests/spec/core/ui-spec.js that raises a string error, clicks the pill, and asserts the modal list renders the message without throwing.

• Fixes respec pill (error) #5355

[marcoscaceres]

@copilot can you fix the CI issue related to the title of the PR?

[Copilot]

@copilot can you fix the CI issue related to the title of the PR?

The CI check fails because the PR title doesn't follow Conventional Commits (needs a type: prefix, e.g. fix:), which amannn/action-semantic-pull-request requires. I don't have a tool to rename the PR title directly, so could you update it to something like:

fix: crash when clicking error/warning pill for string messages

That should satisfy the "Check PR title" workflow.