Refactor FXIOS-15783 Reader mode Render webpages and set up caching for custom scheme

[Alex-Bangu]

📜 Tickets

Jira ticket
Github issue

💡 Description

This pull request sets up page rendering and caching without replacing the legacy localhost implementation just yet.

ReaderModeSchemeHandler.swift:
Single router replaced with normalRouter/privateRouter so private tabs use MemoryReaderModeCache and normal tabs use DiskReaderModeCache. Router selection happens per-request based on websiteDataStore.isPersistent.

PageRoute.swift:
Expanded from a skeleton into a full TinyRoute implementation. Checks cache first via fetchOrExtract, falls back to ReadabilityService extraction on miss, renders via ReaderModeUtils.generateReaderContent, and serves an error page with a "Load Original" link on failure (might need to add a string for this).

ReadabilityService.swift:
Added ReadabilityServiceError enum (.timeout, .noResult) and an async extract method that wraps ReadabilityOperation.

ReaderModeCache.swift:
Added Sendable to the protocol so PageRoute (which conforms to Sendable via TinyRoute) can store it.

PageRouteTests.swift:
8 tests: URL param validation (3), cache-hit rendering (2), cache-miss extraction (1), extraction failure error page (1),
CSP/response builders (2).

MockReaderModeCache.swift:
Mock cache for testing

ReaderModeSchemeHandlerTests.swift:
Updated TODO comment, removed defunct test

🎥 Demos

No user-visible changes to show

📝 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