[core] Combine entity register + configure_entity_ into one call

Add 4-arg overloads to App.register_<entity>() that call configure_entity_()
and push_back in a single function. Codegen defers App.register_<entity>(var)
emission until finalize_entity_strings(), which then emits a single combined
App.register_<entity>(var, name, hash, packed) call instead of the previous
two-statement pair (App.register_X(var); var->configure_entity_(...)).

Apollo R-PRO-1 (ESP32-S3 IDF, 122 components, 164 entities), same toolchain:
  text:     -1248 bytes
  main.cpp: -164 lines

No behavior change. configure_entity_ remains protected on EntityBase; the
Application class is now a friend so the new overloads can call it.

tests/component_tests/binary_sensor/test_binary_sensor.py

@@ -31,7 +31,7 @@ def test_binary_sensor_sets_mandatory_fields(generate_main):
     )
 
     # Then
-    assert 'bs_1->configure_entity_("test bs1",' in main_cpp
+    assert 'App.register_binary_sensor(bs_1, "test bs1",' in main_cpp
     assert "bs_1->set_pin(" in main_cpp