Fix lang param to default to first supported language on page

[GregHolmes]

Description

Fixes a bug where unsupported language query parameters cause all language-specific content to be displayed simultaneously on MDX pages.

Problem

When a user visits an MDX page with a ?lang= query parameter for a language that exists globally but isn't supported on that specific page (e.g., ?lang=python on the statistics page which only supports JavaScript, Ruby, Java, etc.), the following occurred:

1. determineActiveLanguage() would validate the language parameter against the global languageInfo object
2. If the language existed globally, it would return that language even though the page doesn't support it
3. The layout context would then set language: null because the language wasn't in the page's activeLanguages array
4. The <If> component checks if (lang !== undefined && language) - when language is null, this condition becomes false
5. With the language check skipped entirely, shouldShow remains true for all <If> blocks
6. Result: All language-specific content renders simultaneously, creating a confusing mess

[coderabbitai]

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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 EDU-2046-fix-unsupported-language-not-defaulting

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