Middleware file outside allowed location should throw an explicit error

### Link to the code that reproduces this issue https://codesandbox.io/p/devbox/m3wg99 ### To Reproduce 1. Create a new project with Next.js. 2. Create a `middleware.ts` file outside the appropriate folder 3. Run the `next dev` or `next build` command 4. Notice that no specific error or warning is displayed about the incorrect location of the `middleware.ts` file, but a message is generated saying `Compiled middleware in ...`. ### Current vs. Expected behavior Current : When the `middleware.ts` file is placed in an inappropriate location, no specific error or warning is displayed and the project runs incorrectly. Expected : If the `middleware.ts` file is placed in an inappropriate location, a clear error should be displayed at build or dev time, informing the user that the file location needs to be corrected. ### Provide environment information ```bash Operating System: windows 10 Binaries: Node: v20.11.1 pnpm: 8.15.4 npm: 10.2.4 Relevant Packages: next: 15.3.1 react: 19.0.0 react-dom: 19.0.0 typescript: 5 ``` ### Which area(s) are affected? (Select all that apply) Middleware ### Which stage(s) are affected? (Select all that apply) next dev (local), next build (local) ### Additional context - This change can help to fix issues in larger projects that may have complex folder structures. - You may also want to suggest that Next.js automatically stop the project if middleware files are misplaced during build time to prevent problems.