incomplete go.sum file in 4.0.0 branch

```
go mod vendor
go: boringssl.googlesource.com/boringssl/crypto/ecdh_extra imports
github.com/ethereum/go-ethereum/crypto/secp256k1: missing go.sum entry for module providing package github.com/ethereum/go-ethereum/crypto/secp256k1 (imported by boringssl.googlesource.com/boringssl/crypto/ecdh_extra); to add:
go get boringssl.googlesource.com/boringssl/crypto/ecdh_extra
```

In the 4.0.0 branch, start by running `go mod vendor` and review the resulting missing `go.sum` entry for `github.com/ethereum/go-ethereum/crypto/secp256k1`. Check the module metadata involved in the error, then verify that dependency resolution and `go mod vendor` complete without the missing-entry error.

Written by the indexing model from the issue text.