Implement fallback handling for variant space navigation

[conico974]

Fix resolution of the navigation between variants when the pathname is different.

It uses metalink as a fallback to find an alternative page to redirect to when the pathname does not exist in the destination variant

[linear]

RND-9299 Switching to a different language brings the user back to the root of the space

[changeset-bot]

⚠️ No Changeset found

Latest commit: 4768d53

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[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel	https://gitbook-v2-oyo5uausa-gitbook.vercel.app	✅
Cloudflare	https://b64ecab6-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	2v	2c
GitBook	https://gitbook-v2-oyo5uausa-gitbook.vercel.app/url/gitbook.com/docs	https://b64ecab6-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.com/docs
E2E	https://gitbook-v2-oyo5uausa-gitbook.vercel.app/url/gitbook.gitbook.io/test-gitbook-open	https://b64ecab6-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.gitbook.io/test-gitbook-open

[argos-ci]

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Build	Status	Details	Updated (UTC)
customers-v2 (Inspect)	⚠️ Changes detected (Review)	29 changed	Jan 15, 2026, 9:36 AM
v2-cloudflare (Inspect)	⚠️ Changes detected (Review)	14 changed	Jan 15, 2026, 9:41 AM
v2-vercel (Inspect)	⚠️ Changes detected (Review)	33 changed	Jan 15, 2026, 9:40 AM

[SamyPesse]

Blocking until I can review this one this afternoon