fix(template-no-nested-interactive): More useful error message

[johanrd]

Summary

The current template-no-nested-interactive error message gives only the bare tag of each side:

Do not nest interactive element <button> inside <div>.

When the parent is a <div> (or <span>, etc.) the message hides why it was classified as interactive — authors have to inspect the rule source or work backwards from INTERACTIVE_ROLES / isHtmlInteractiveContent to figure out that role, contenteditable, or tabindex triggered the rule.

This PR makes the message name the disambiguating attribute when the bare tag would be uninformative:

Do not nest interactive element <button> inside <div role="menu">.
Do not nest interactive element <input> inside <div contenteditable>.
Do not nest interactive element <div tabindex="1"> inside <button>.
Do not nest interactive element <div role="button"> inside <button>.

Self-explanatory native interactive tags (button, input, a, label, select, textarea, details, summary, embed, iframe, audio[controls], video[controls], img[usemap], canvas, object[usemap]) keep their bare-tag form, since the tag itself already conveys interactivity. Applied symmetrically on both the parent and child sides.

The change is purely diagnostic — no rule firing/non-firing decisions are altered.

Implementation

• New describeInteractive(node) helper at module scope. Surfaces:
  • role="X" when X is in INTERACTIVE_ROLES
  • contenteditable (and contenteditable="plaintext-only" when that exact spec keyword is used; '' / 'true' / bare attribute collapse to contenteditable)
  • tabindex="X" when interactivity comes from tabindex on a tag that isn't otherwise interactive
• Stored as describe on each interactiveStack entry at push time, used as data.parent.
• Computed fresh for the child at report time, used as data.child.

Test impact

• 99 existing tests pass.
• One existing message assertion updated (<button><div tabindex="1"></div></button> → child now says <div tabindex="1"> instead of <div>).
• New invalid cases added covering role="menu" parent (regression test for the original report), contenteditable parent, contenteditable="plaintext-only" parent, and role="button" child enrichment.

Test plan

• pnpm exec vitest run tests/lib/rules/template-no-nested-interactive.js — 99/99 pass
• Verified MDN-canonical menu pattern (<ul role="menu"><li role="presentation"><a role="menuitem" href>) still passes the rule
• Verified <li role="menuitem"><button> correctly fires with the new message naming the parent's role