AddNpmApp does not build Next.js app before starting — missing '.next' build directory

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

When using builder.AddNpmApp(...) with a Next.js frontend project (in production mode), Aspire fails to run the app because the production build is missing. The startup log shows:

`[Error: Could not find a production build in the '.next' directory. Try building your app with 'next build' before starting the production server. https://nextjs.org/docs/messages/production-start-no-build-id]
`

Even though WithNpmPackageInstallation() is present, it seems that next build is not executed automatically before npm start.

### Expected Behavior

Aspire should support typical production workflows for popular frontend frameworks like Next.js. If `npm start `requires next build, Aspire should either:

run `npm run build` automatically before npm start (when in production mode), or

encourage developers to specify `.WithBuildCommand("npm run build") `explicitly in documentation.

### Steps To Reproduce

builder.AddNpmApp("frontend", "../frontend")
.WithEnvironment("NODE_ENV", "production")
.WithNpmPackageInstallation()
.WithHttpEndpoint(env: "PORT")
.PublishAsDockerFile();

### Exceptions (if any)

[Error: Could not find a production build in the '.next' directory. Try building your app with 'next build' before starting the production server. https://nextjs.org/docs/messages/production-start-no-build-id]

### .NET Version info

_No response_

### Anything else?

.NET SDK:
Version: 9.0.301
Commit: a596cd22e2
Workload version: 9.0.300-manifests.07067f5b
MSBuild version: 17.14.5+edd3bbf37