fix(a11y): replace aria-hidden button with role=presentation span in EntityTreeNode

[R-Hart80]

Summary

• Replaces the <button> expand/collapse chevron in EntityTreeNode with a <span role="presentation">
• Removes the conflicting tabIndex={-1} and aria-hidden="true" combination that caused Chrome DevTools to flag: "Blocked aria-hidden on an element because its descendant retained focus"

Why this fix works

A <button> can still receive focus programmatically (e.g. via mouse click) even with tabIndex={-1}, which contradicts aria-hidden="true". A <span role="presentation"> removes the element from the accessibility tree entirely while preserving click functionality for mouse users. Keyboard navigation is already handled by the parent <li role="treeitem">.

Test plan

• Open the editor and expand/collapse tree nodes — chevron should still work on click
• Run Chrome DevTools accessibility audit — no more aria-hidden focus conflict warning
• Tab through the tree with keyboard — expand/collapse is handled by the parent li, no change in behavior

Fixes #100

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[JohnRDOrazio]

Closing in favor of #153, which includes this fix along with #144's fix, plus additional improvements (cursor-pointer class, data-testid attribute, and updated tests).