[Bug] aria-hidden element with focusable descendants breaks the accessibility tree on API reference pages (+ heading order & default-theme contrast)

[morguama]

Theme: prism · Layout: sidenav
Where: OpenAPI-generated API reference pages (e.g. /api-reference/permits/search-permits), desktop + mobile — identical results.

1. aria-hidden="true" contains focusable descendants → malformed accessibility tree. The data-agent-docs-index element (blockquote.sr-only) is set aria-hidden="true" but contains focusable descendants. Lighthouse flags this both as an Accessibility failure and as "Accessibility tree is not well-formed" (the Agentic Browsing / AI-readability audit). Fixing this one issue resolves both. Expected: remove focusable descendants from the aria-hidden subtree, or don't apply aria-hidden to a container holding focusable content.

2. Heading order skips levels. API section headings render as <h4 class="api-section-heading-title"> with no preceding <h2>/<h3>, so heading order skips levels. Expected: a proper heading hierarchy in the OpenAPI template.

3. Insufficient color contrast on default theme chrome (lower priority). The search bar (#search-bar-entry), AI-assistant button (#assistant-entry), and the green GET method pill (#2AB673) fail WCAG AA against their backgrounds. These aren't exposed in mint.json, so site owners can't fix them.

Impact: Accessibility 92 and Agentic Browsing 67 on pages that otherwise score ~100 on Performance/Best Practices/SEO. The agentic/AI-readability score is especially relevant since coding agents are primary consumers of API docs.
Audit: Chrome DevTools + Lighthouse, desktop + mobile.