Refactor fullscreen rejection handling to use WebIDL react pattern

[marcoscaceres]

This commit modernizes how the Screen Orientation specification responds to fullscreen request rejections by using the WebIDL 'react' concept instead of the previous flag-based approach.

Depends on whatwg/fullscreen#251

Changes:

• Replace flag-based 'pending fullscreen request flag unset due to rejection' pattern
• Use WebIDL 'react' pattern with 'pending fullscreen request promise'
• Update reference to use proper 'pending fullscreen request flag' terminology
• Maintain all existing algorithm behavior while modernizing the specification architecture

This enables cleaner, more maintainable specification text that follows modern WebIDL patterns for promise-based APIs. The change complements the fullscreen spec changes in whatwg/fullscreen that add both the exported flag and promise definitions needed for this integration.

Closes #254
Closes #255

The following tasks have been completed:

• Modified Web platform tests (link to pull request)

Implementation commitment:

• WebKit (https://bugs.webkit.org/enter_bug.cgi)
• Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
• Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)

---

Preview | Diff

[Copilot]

Pull Request Overview

This PR refactors the Screen Orientation specification to use the modern WebIDL 'react' pattern for handling fullscreen request rejections, replacing the previous flag-based approach. The change enables cleaner specification text while maintaining all existing algorithm behavior and better alignment with modern WebIDL patterns.

Key Changes:

• Replaces implicit flag-based fullscreen rejection handling with explicit WebIDL 'react' pattern
• Expands the fullscreen pre-lock condition definition from a simple reference to a detailed algorithmic specification
• Adds explicit handling for rejecting orientation lock promises when fullscreen requests are rejected or when documents exit fullscreen

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

Pull Request Overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}