feat: validate values for cache-control and content-type headers in dev mode

[JR-G]

Fix for #12784

Adds validation for common HTTP headers in dev mode to help catch invalid values early:

• Validates cache-control directives against standard values
• Validates content-type format
• Only active in dev mode

run pnpm dev from test/apps/basics and navigate to http://localhost:5173/headers/invalid. In your console you'll see:

[SvelteKit] Invalid cache-control directive "totally-invalid". Did you mean one of: max-age, public, private, no-cache, no-store, must-revalidate, proxy-revalidate, s-maxage, immutable, stale-while-revalidate, stale-if-error, no-transform, only-if-cached, max-stale, min-fresh
[SvelteKit] Invalid content-type value "not-a-real-type"

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🦋 Changeset detected

Latest commit: adc6077

The changes in this PR will be included in the next version bump.

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Name	Type
@sveltejs/kit	Minor

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13114-svelte.vercel.app/

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