fix(a11y): add skip-to-operations link, banner and main landmarks

[bmatar]

Description

Adds three structural a11y improvements:

1. <main id="operations"> wraps the operations + webhooks + models region in BaseLayout. Provides a primary landmark for Screen Reader (SR) navigation in every swagger-ui consumer (standalone or custom layout). tabIndex="-1" + outline:none keeps it focusable programmatically (so the skip link can move focus to it) without adding a tab stop.

2. <header role="banner"> wraps the Topbar in StandaloneLayout. SR users can jump to it via banner navigation.

3. Skip-to-operations link in StandaloneLayout — visually hidden until focused, then appears top-left as a high-contrast button. The click handler scopes the target lookup via closest('.swagger-ui') so the link works correctly when multiple swagger-ui instances are mounted on one page.

Motivation and Context

The default layout currently has no <main> landmark. Standalone has no banner and no bypass mechanism. Keyboard and SR users must Tab through the topbar's logo, filter input, and dark-mode toggle on every navigation before reaching the operations list — and have no landmark structure to navigate via SR shortcut keys.

Refs #7350 (umbrella accessibility issue). WCAG 2.4.1 (Bypass Blocks), 1.3.1 (Info and Relationships).

Detected by Tactual.

How Has This Been Tested?

• npm run test:unit — all suites pass

• npm run build — clean

• DOM probe (master vs this branch):

  BEFORE (master)
    <main>          : not present
    <header banner> : not present
    skip link       : not present
  
  AFTER (this branch)
    <main id="operations" tabindex="-1">   : wraps operations/webhooks/models
    <header role="banner">                 : wraps Topbar (StandaloneLayout only)
    <a class="swagger-ui__skip-link" href="#operations">Skip to operations</a>
                                           : prepended in StandaloneLayout
  

• Keyboard flow on the local build:

  • First Tab → focus lands on .swagger-ui__skip-link (visible top-left)
  • Enter → focus moves to <main id="operations">

• Manually verified with NVDA + Chrome on Windows 11:

  • Browse-mode landmark navigation exposes the topbar as a banner landmark (initially NVDA may announce nearby interactive context as "clickable banner landmark ..."; returning via landmark navigation announces "banner")
  • First Tab: skip link visible, announced by name
  • Enter on skip link: focus moves to <main>; NVDA announces the main landmark and then the first operation heading/link
  • Landmark quick navigation moves between the banner and main landmarks

Screenshots

The skip link is visually hidden until focused. After pressing Tab once on page load:

BEFORE (master)	AFTER (this branch)

No other visual change.

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.