Load the KVM ssh material on first use instead of at startup

The kvm-ssh Secret is mounted optional, but kvm.New read the key and
known_hosts eagerly and registry.Build aborts the whole provider set on a
constructor error, so a missing or misnamed Secret exited the manager
before any controller started and took the kubernetes and gcp providers
down with it.

lazyAuth loads the material on the first connection and caches it once it
works, so a Secret created after the manager starts heals the affected
proxies on the next reconcile with no restart. A missing file classifies
transient for that reason; a key that does not parse is permanent.

New still checks both files and logs a WARNING: line when they are
unreadable, keeping the startup diagnostic the eager load was there for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/provider/kvm/kvm_test.go

@@ -906,12 +906,20 @@ func TestMapState(t *testing.T) {
 	}
 }
 
-func TestNew_badKeyPathFailsFast(t *testing.T) {
+// A bad key path no longer fails the constructor: registry.Build aborts the
+// whole provider set on a constructor error, so that took the manager down
+// with every other provider. It degrades per-proxy instead — see
+// TestNew_missingSSHMaterialStillBuilds and TestLazyAuth_healsWhenTheKeyAppears.
+func TestNew_badKeyPathDefersToFirstUse(t *testing.T) {
 	t.Parallel()
 	cfg := testConfig("nas1")
 	cfg.KVM.SSHKeyFile = t.TempDir() + "/missing"
-	if _, err := New(context.Background(), cfg); err == nil {
-		t.Fatal("New succeeded with a missing key file")
+	p, err := New(context.Background(), cfg)
+	if err != nil {
+		t.Fatalf("New() error = %v, want a provider that fails only when used", err)
+	}
+	if err := p.Delete(context.Background(), "hosts/nas1/domains/proxy-x"); provider.Class(err) != provider.ErrTransient {
+		t.Errorf("Delete class = %v, want ErrTransient", provider.Class(err))
 	}
 }