fix: normalize file path before creating route ID in configRoutesToRouteManifest

[restareaByWeezy]

Problem

Fixes #14125

When routes are defined using the relative() helper in routes.ts, route IDs become absolute file paths (e.g., /home/projects/app/routes/home) instead of the expected relative paths (e.g., routes/home).

This happens because relative() calls Path.resolve(directory, file) to produce absolute paths, and configRoutesToRouteManifest passes this absolute path directly to createRouteId() without normalizing it first.

Root Cause

In configRoutesToRouteManifest's walk() function, the file property was already normalized from absolute to relative:

file: Path.isAbsolute(route.file)
  ? Path.relative(appDirectory, route.file)
  : route.file,

But createRouteId(route.file) on the line above used the raw route.file, which is still absolute when coming from relative().

Solution

Extract the file normalization into a shared normalizedFile variable so both the route ID and the file property use the relative path.

let normalizedFile = Path.isAbsolute(route.file)
  ? Path.relative(appDirectory, route.file)
  : route.file;
let id = route.id || createRouteId(normalizedFile);

This is a minimal change that applies the existing normalization logic to the ID computation.

Changeset

Included a patch changeset for @react-router/dev.

Test Plan

• Verified the logic: relative() produces absolute paths via Path.resolve(), and the fix ensures createRouteId receives a relative path, matching the behavior of non-relative() routes
• No existing unit tests for configRoutesToRouteManifest in the repo; the fix is a straightforward refactor of existing normalization logic

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[remix-cla-bot]

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