fix: consider oidc session events for authN milestones (#8089)

# Which Problems Are Solved

After migrating the access token events in #7822, milestones based on
authentication, resp. theses events would not be reached.

# How the Problems Are Solved

Additionally use the `oidc_session.Added` event to check for
`milestone.AuthenticationSucceededOnInstance` and
`milestone.AuthenticationSucceededOnApplication`.

# Additional Changes

None.

# Additional Context

- relates to #7822 
- noticed internally

internal/api/grpc/system/instance_integration_test.go

@@ -14,7 +14,7 @@ import (
 )
 
 func TestServer_ListInstances(t *testing.T) {
-	domain, instanceID, _ := Tester.UseIsolatedInstance(t, CTX, SystemCTX)
+	domain, instanceID, _, _ := Tester.UseIsolatedInstance(t, CTX, SystemCTX)
 
 	tests := []struct {
 		name    string