fix: set basePath from next config as env variable (#15154)

### What
Fix image preview URLs not displaying when `basePath` is set in
`next.config.mjs`.

### Why
When `basePath` is set in `next.config.mjs`, the `withPayload` function
assigns it as an env variable. However, currently the file sets
`basePath` as `baseConfig.env.NEXT_BASE_PATH`, which is only accessible
in Next.js runtime contexts (API routes, server components).

Upload URLs are generated from `formatAdminURL` in the Node.js
environment, where `baseConfig.env.NEXT_BASE_PATH` cannot be accessed.
As a result, it thinks no `basePath` is set and uses the incorrect URL
to render the upload preview.

### Where
`withPayload` now sets both `baseConfig.env.NEXT_BASE_PATH` **and**
`process.env.NEXT_BASE_PATH`, making `basePath` available everywhere.

Previously, users had to manually set `NEXT_BASE_PATH` in `.env` to work
around this.

##### Fixes #15111

Author: JessRynkar

packages/next/src/withPayload/withPayload.js

@@ -208,6 +208,7 @@ export const withPayload = (nextConfig = {}, options = {}) => {
   }
 
   if (nextConfig.basePath) {
+    process.env.NEXT_BASE_PATH = nextConfig.basePath
     baseConfig.env.NEXT_BASE_PATH = nextConfig.basePath
   }
 

packages/next/src/withPayload/withPayload.spec.ts

@@ -0,0 +1,48 @@
+import { describe, expect, it } from 'vitest'
+
+import { withPayload } from './withPayload.js'
+
+describe('withPayload', () => {
+  it('should set process.env.NEXT_BASE_PATH when nextConfig.basePath is provided', () => {
+    const originalBasePath = process.env.NEXT_BASE_PATH
+    delete process.env.NEXT_BASE_PATH
+
+    try {
+      const mockNextConfig = {
+        basePath: '/test/basepath',
+      }
+
+      withPayload(mockNextConfig)
+
+      // Verify it set the env var so formatAdminURL can read it
+      expect(process.env.NEXT_BASE_PATH).toBe('/test/basepath')
+    } finally {
+      // Restore original value
+      if (originalBasePath === undefined) {
+        delete process.env.NEXT_BASE_PATH
+      } else {
+        process.env.NEXT_BASE_PATH = originalBasePath
+      }
+    }
+  })
+
+  it('should not modify process.env.NEXT_BASE_PATH when basePath is not provided', () => {
+    const originalBasePath = process.env.NEXT_BASE_PATH
+
+    try {
+      const mockNextConfig = {}
+
+      withPayload(mockNextConfig)
+
+      // Verify it didn't set the env var
+      expect(process.env.NEXT_BASE_PATH).toBe(originalBasePath)
+    } finally {
+      // Restore original value
+      if (originalBasePath === undefined) {
+        delete process.env.NEXT_BASE_PATH
+      } else {
+        process.env.NEXT_BASE_PATH = originalBasePath
+      }
+    }
+  })
+})