fix: Rename proxy.js to middleware.js in NFT file (#86214)
### Why?
When Proxy self-references itself, e.g., `__filename`, it is traced to
the NFT file. However, since Next.js renames 'proxy.js' to
'middleware.js' during webpack build, the files in NFT will differ from
the actual outputs. This can cause a problem when verifying the NFT.
Current Behavior:
https://github.com/vercel/next.js/actions/runs/19483162554/job/55760669267?pr=86214#step:34:91
### How?
Therefore, also rename the self-reference of `'proxy.js'` in
`middleware.js.nft.json` to `'middleware.js'`.
I don't expect other modules to be referencing the build output of
Proxy, that is not recommended.
Closes NEXT-4777
Closes NAR-518
---------
Co-authored-by: Niklas Mischkulnig <4586894+mischnic@users.noreply.github.com>
