Redirect to space base path on fallback true

[conico974]

Implement a redirect to the space base path instead of returning a 404 when a page is not found and fallback mode is active (i.e. when switching variants)
This change enhances user experience by guiding users to the base path rather than displaying the not found page first and then redirecting.

[linear]

RND-8444 Redirect pages with ?fallback query parameter with 307 temporary redirect

[changeset-bot]

⚠️ No Changeset found

Latest commit: 3f4b349

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel	https://gitbook-v2-kfw1trt20-gitbook.vercel.app	✅
Cloudflare	https://c2153c6b-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	2v	2c
GitBook	https://gitbook-v2-kfw1trt20-gitbook.vercel.app/url/gitbook.com/docs	https://c2153c6b-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.com/docs
E2E	https://gitbook-v2-kfw1trt20-gitbook.vercel.app/url/gitbook.gitbook.io/test-gitbook-open	https://c2153c6b-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)	✅ No changes detected	-	Oct 21, 2025, 9:57 AM
v2-cloudflare (Inspect)	✅ No changes detected	-	Oct 21, 2025, 10:01 AM
v2-vercel (Inspect)	👍 Changes approved	2 changed	Oct 21, 2025, 9:59 AM