fix: preserve array query parameters in dynamic group redirects

[jaydeep-pipaliya]

Summary

• Fixes incorrect serialization of array query parameters during dynamic group redirects
• Replaces new URLSearchParams(context.query as Record<string, string>) with encode(context.query) from node:querystring (already imported in the file)

What was wrong

context.query in Next.js is typed as Record<string, string | string[] | undefined>. Casting it to Record<string, string> and passing to URLSearchParams causes array values like ?user=john&user=doe to be serialized as user=john,doe — a single comma-joined value instead of repeated keys.

Fix

node:querystring.encode() natively handles string | string[] values, producing user=john&user=doe for arrays. It was already imported at line 1 but unused for this code path.

Test plan

• URL with repeated query params (/john?user=john&user=doe) preserves both values in redirect
• URL with single query params continues to work unchanged
• Dynamic group redirect includes all original query parameters

Fixes #28687

[github-actions]

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[romitg2]

please re-open with before and after video demo attached.