feat: MustServiceFor[T] + fix service names test for auto-registered cli

Co-Authored-By: Virgil <virgil@lethean.io>

service.go

@@ -140,6 +140,18 @@ func ServiceFor[T any](c *Core, name string) (T, bool) {
 	return typed, ok
 }
 
+// MustServiceFor retrieves a registered service by name and asserts its type.
+// Panics if the service is not found or the type assertion fails.
+//
+//	cli := core.MustServiceFor[*Cli](c, "cli")
+func MustServiceFor[T any](c *Core, name string) T {
+	v, ok := ServiceFor[T](c, name)
+	if !ok {
+		panic(E("core.MustServiceFor", Sprintf("service %q not found or wrong type", name), nil))
+	}
+	return v
+}
+
 // Services returns all registered service names.
 //
 //	names := c.Services()

service_test.go

@@ -47,9 +47,9 @@ func TestService_Names_Good(t *testing.T) {
 	c.Service("a", Service{})
 	c.Service("b", Service{})
 	names := c.Services()
-	assert.Len(t, names, 2)
 	assert.Contains(t, names, "a")
 	assert.Contains(t, names, "b")
+	assert.Contains(t, names, "cli") // auto-registered by CliRegister in New()
 }
 
 // --- Service Lifecycle ---