fix: prepend base path when redirecting

[denesbeck]

GH issue: #85267

Summary

Add basePath support to redirect functions

Changes

This PR modifies the redirect() and permanentRedirect() functions to automatically prepend the basePath (configured in next.config.js) to relative redirect URLs.

Implementation

• Added getBasePath() function: Retrieves the basePath from process.env.__NEXT_ROUTER_BASEPATH
• Added prependBasePath() helper: Safely combines basePath with redirect URLs while handling edge cases:
  • Skips absolute URLs (with http:// or https://)
  • Prevents duplicate basePath prepending
  • Handles trailing/leading slashes correctly
• Modified getRedirectError(): Now prepends basePath to the URL before creating the redirect error

[ijjk]

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