Expose edge runtime fields in build-complete (#91239)
### What?
- Add canonical edge entrypoint metadata to edge function definitions in
the middleware manifest.
- Surface that canonical entrypoint through `build-complete` as
`edgeRuntime` metadata in adapter outputs.
- Update middleware and adapter tests to assert the new metadata instead
of relying on bundler-specific emitted file names.
### Why?
Adapter consumers need a stable way to load and invoke edge outputs
without depending on `next-server`'s sandbox handling or guessing which
emitted file is the executable entrypoint. The existing checks were also
brittle across webpack and Turbopack because the emitted file lists can
differ.
### How?
- Add an optional `entrypoint` field to `EdgeFunctionDefinition` and
populate it in both the webpack middleware manifest generator and the
Turbopack manifest emitters for app, pages, and middleware edge
functions.
- In `packages/next/src/build/adapter/build-complete.ts`, use that
canonical entrypoint for `filePath` and expose `edgeRuntime.modulePath`,
`edgeRuntime.entryKey`, and `edgeRuntime.handlerExport` for edge
outputs.
- Adjust the middleware manifest tests to validate cross-bundler
invariants and add adapter-config coverage for the new `edgeRuntime`
fields.
