test(network/grpc): fix race in Test_grpcConnectionManager_Peers

[stevenvegt]

Problem

`Test_grpcConnectionManager_Peers/inbound_stream_triggers_observer` (and the mirror outbound subtest) is flaky on CI. Example failure: PR #4275, run 26294161548.

The failure message:

```
controller.go:97: missing call(s) to *grpc.MockAuthenticator.Authenticate(is equal to did:nuts:test (did.DID), is anything)
```

Root cause

Both subtests set up two peers (cm1 and cm2) with separate `MockAuthenticator` mocks, and expect `Authenticate` to be called on both:

```go
authenticator1.EXPECT().Authenticate(*nodeDID, gomock.Any()).Return(transport.Peer{ID: "1"}, nil)
authenticator2.EXPECT().Authenticate(*nodeDID, gomock.Any()).Return(transport.Peer{ID: "2"}, nil)
```

But the wait condition only watches one side's observer:

```go
test.WaitFor(t, func() (bool, error) {
return capturedPeer.Load() != nil, nil
}, time.Second*2, "waiting for peer 1 observers")
```

Both authentications happen in parallel on opposite ends of the bufconn. When the observed side wins the race, `cm2.Stop()` runs while the other side's `Authenticate` is still in flight, and `t.Cleanup` triggers gomock's `Finish()` before the mock call lands.

Fix

Extend the wait condition to also require the unobserved side's `Peers()` list to be non-empty. A side only adds a remote to its `Peers()` list after its own `Authenticate` returns, so this guarantees both expectations have been satisfied before teardown.

Test plan

• `go test -count=20 -run "Test_grpcConnectionManager_Peers" ./network/transport/grpc/` passes locally (20/20)
• CI passes

[qltysh]

Coverage Impact

⬇️ Merging this pull request will decrease total coverage on master by 0.02%.

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