Add FXIOS-15848 [Add Shortcut Tile] Implement add shortcuts tile

[Foxbolts]

📜 Tickets

Jira ticket
Github issue

💡 Description

• Implement "Add Shortcut" tile to the shortcuts section and shortcuts library

📝 Technical Notes:

• The "Add Shortcut" tile shows at the end of homepage shortcuts section (first tile after the last webpage shortcut) only when there is room, it is otherwise displaced to the shortcuts library (meaning, webpage shortcuts take priority). It cannot be displaced out of the shortcuts library, it will always be the last tile in the shortcuts library list.
• Shortcuts section header title with shortcuts library affordance will display, even if the only additional shortcut in the shortcuts library is the "Add Shortcut" tile. If the "Add Shortcut" tile is presented on the homepage, the shortcuts section header will never be shown.
• The "Add Shortcut" tile currently has no tap functionality (will be implemented in FXIOS-15849)
• The "Add Shortcut" tile has no long-press actions

🎥 Demos

Before	After

📝 Checklist

• I filled in the ticket numbers and a description of my work
• I updated the PR name to follow our PR naming guidelines
• I ensured unit tests pass and wrote tests for new code
• If working on UI, I checked and implemented accessibility (Dynamic Text and VoiceOver)
• If adding telemetry, I read the data stewardship requirements and will request a data review
• If adding or modifying strings, I read the guidelines and will request a string review from l10n
• If needed, I updated documentation and added comments to complex code

[mobiletest-ci-bot]

	Warnings
⚠️	❌ Existing file code coverage The following file(s) are below their scaled coverage: File Coverage Required firefox-ios/Client/Frontend/Home/Homepage/TopSites/TopSiteCell.swift 0.0% 23.3% Bypass label ignore-code-coverage detected — reporting as warnings only for this PR.

	Messages
📖	Project coverage: 42.29%

💪 Quality guardian

5 tests files modified. You're a champion of test coverage! 🚀

🧩 Neat Piece

This PR changes 415 lines. It's a substantial update,
but still review-friendly if there’s a clear description. Thanks for keeping things moving! 🚀

💬 Description craftsman

Great PR description! Reviewers salute you 🫡

✅ New file code coverage

No new file detected so code coverage gate wasn't ran.

Client.app: Coverage: 40.73

File	Coverage
HomepageLayoutMeasurementCache.swift	0.0%	⚠️
HomepageDiffableDataSource.swift	81.56%	✅
HomepageSectionLayoutProvider.swift	29.38%	⚠️
TopSitesMiddleware.swift	86.31%	✅
ShortcutsLibraryState.swift	80.0%	✅
TopSitesAction.swift	100.0%	✅
TopSitesSectionState.swift	97.69%	✅
TopSiteCell.swift	0.0%	⚠️
ShortcutsLibraryDiffableDataSource.swift	75.76%	✅
ShortcutsLibraryViewController.swift	55.35%	✅
HomepageViewController.swift	47.03%	⚠️

Generated by 🚫 Danger Swift against 56c191a

[Foxbolts]

@mozilla-mobile/fxios-unit-test-owners TopSiteCell is mostly UI code without much business logic to test.