Affected: Go: github.com/coredns/coredns

### Summary

The gRPC, QUIC, DoH, and DoH3 transports in CoreDNS incorrectly handle TSIG authentication.

For gRPC and QUIC, CoreDNS checks whether the TSIG key name exists in the config, but does not actually verify the TSIG HMAC. If the key name matches, `tsigStatus` remains nil and the tsig plugin treats the request as "verified".

For DoH and DoH3, the issue is worse: TSIG is not verified at all. The DoH response writer has `TsigStatus()` hardcoded to return nil, so any request containing a TSIG record is treated as authenticated, even if the key name is invalid and the MAC is garbage.

As a result, attackers may bypass TSIG authentication on affected transports and access TSIG-protected functionality such as AXFR/IXFR zone transfers, dynamic updates, or other TSIG-gated plugin behavior.

### Details

In `server_grpc.go` and `server_quic.go`, the TSIG handling checks whether the TSIG key name exists, but does not call `dns.TsigVerify()`.

Relevant code before fix:

```go if tsig := msg.IsTsig(); tsig != nil { if s.tsigSecret == nil { w.tsigStatus = dns.ErrSecret } else if _, ok := s.tsigSecret[tsig.Hdr.Name]; !ok { w.tsigStatus = dns.ErrSecret } // key found -> nothing happens -> tsigStatus stays nil -> "verified" } ```

This means that for gRPC and QUIC, a request with a known TSIG key name but an invalid MAC is accepted as authenticated.

PRs #7943 and #7947 partially addressed this area by adding key name checks for gRPC and QUIC, but d…