Reject setting both OAuth token-URL closures

A Registration that sets both BuildTokenURLForDomain and
BuildTokenURLForSite would pass startup but silently use only the first
at the OAuth callback (CompleteWithState checks them in order). Reject
the combination in Register so the misconfiguration fails loud at
process start, matching the existing APIKeyBasicAuth/APIKeyHeader guard.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/connector/provider/registry_test.go

@@ -109,6 +109,20 @@ func TestRegistry_Register(t *testing.T) {
 		assert.Contains(t, err.Error(), "mutually exclusive")
 	})
 
+	t.Run("BuildTokenURLForDomain and BuildTokenURLForSite mutually exclusive", func(t *testing.T) {
+		t.Parallel()
+
+		r := provider.NewRegistry()
+		err := r.Register(&provider.Registration{
+			Provider:               coredata.ConnectorProviderSlack,
+			DisplayName:            "Slack",
+			BuildTokenURLForDomain: func(string) (string, error) { return "", nil },
+			BuildTokenURLForSite:   func(string) (string, error) { return "", nil },
+		})
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "mutually exclusive")
+	})
+
 	t.Run("duplicate registration", func(t *testing.T) {
 		t.Parallel()