test(a11y): add structured markup audit harness

[XingYu-Zhong]

Summary

• Rebuilds the useful accessibility-test idea from test(a11y): add static accessibility harness #926 on the current develop branch.
• Adds a structured, deterministic markup audit for component tests without presenting it as a replacement for browser or assistive-technology testing.

Changes

• Parses server-rendered HTML with parse5 instead of matching markup with regular expressions.
• Detects duplicate IDs, every missing aria-labelledby reference, unnamed interactive controls, form-label gaps, modal-dialog gaps, and custom keyboard-semantics gaps.
• Ignores hidden implementation subtrees and exercises a real production component (RuntimeBanner).

Tests

• npx vitest run src/renderer/src/testing/accessibility-harness.test.ts (5 passed).
• Full root app suite (4488 passed) plus all extension-package suites (58 passed).
• Root typecheck, targeted ESLint, and git diff --check.

Original contribution and test direction: @luoye520ww in #926.

Part of #885. Replaces #926.