routing_plugin: release with a live context

`ReleaseRoutingPlugin` called `Done` with the caller context. In the
loop-out payment flow this cleanup is deferred, so the context may already be
canceled by the time teardown runs.

That can prevent mission-control restoration in `Done`, leaving
plugin-induced routing state behind after a swap exits.

Fix this by detaching cancellation in `ReleaseRoutingPlugin` and passing
`context.WithoutCancel(ctx)` to `Done`, so teardown still executes during
caller cancellation.

Also add a regression test that cancels the caller context before release and
asserts the plugin `Done` call still receives a live context.

routing_plugin_test.go

@@ -692,3 +692,51 @@ func TestRoutingPluginAcquireRelease(t *testing.T) {
 	require.NotNil(t, plugin2)
 	require.NoError(t, err)
 }
+
+// mockRoutingPlugin is a minimal RoutingPlugin used to capture the context
+// passed to Done.
+type mockRoutingPlugin struct {
+	doneCtxErr error
+}
+
+// Init is a no-op initializer for the mock plugin.
+func (m *mockRoutingPlugin) Init(_ context.Context, _ route.Vertex,
+	_ [][]zpay32.HopHint, _ btcutil.Amount) error {
+
+	return nil
+}
+
+// Done records ctx.Err() so tests can assert whether teardown ran with a live
+// context.
+func (m *mockRoutingPlugin) Done(ctx context.Context) error {
+	m.doneCtxErr = ctx.Err()
+	return nil
+}
+
+// BeforePayment is a no-op hook for the mock plugin.
+func (m *mockRoutingPlugin) BeforePayment(_ context.Context, _, _ int) error {
+	return nil
+}
+
+// TestReleaseRoutingPluginUsesLiveContext checks that ReleaseRoutingPlugin does
+// not propagate caller cancellation to plugin teardown. The test cancels the
+// caller context before release and verifies mock Done still sees nil ctx.Err.
+func TestReleaseRoutingPluginUsesLiveContext(t *testing.T) {
+	ReleaseRoutingPlugin(context.Background())
+	t.Cleanup(func() {
+		ReleaseRoutingPlugin(context.Background())
+	})
+
+	mockPlugin := &mockRoutingPlugin{}
+
+	routingPluginMx.Lock()
+	routingPluginInstance = mockPlugin
+	routingPluginMx.Unlock()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	ReleaseRoutingPlugin(ctx)
+
+	require.NoError(t, mockPlugin.doneCtxErr)
+}