feat: Add dual language selector for AI Transport guides

[GregHolmes]

Summary

Adds dual language selection for AI Transport guide pages, allowing users to independently choose Client (frontend) and Agent (backend) languages for code examples.

Changes

• Dual language selector UI: Two dropdowns ("Client" and "Agent") replace the single language selector on AI Transport guide pages
• URL param support: ?fe_lang=xxx&be_lang=yyy for bookmarking and sharing specific language combinations
• Navigation preservation: Language params persist when navigating between guide pages
• Code block prefixes: MDX files use fe_ and be_ prefixes (e.g., fe_javascript, be_python) to distinguish frontend vs backend code
• aiTransport languages: javascript, python, swift, java with SDK versions

Supported languages

• Client: JavaScript, Swift, Java
• Agent: JavaScript, Python, Java

Dependencies

• Requires @ably/ui@17.13.1-dev.c839343a (or later) with fe_/be_ prefix support in CodeSnippet
• Also removes stale CookieMessage CSS import that was breaking builds with newer ably-ui versions

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch AIT-108-Framework-language-feature-combos-investigate-and-design-navigation

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