[core] Make register_component protected, remove runtime checks

Instead of runtime null, duplicate, and capacity checks in
register_component_, make the method unreachable from outside
codegen by removing the public template wrapper and granting
friend access to the codegen-generated ::setup() function.

Since ESPHOME_COMPONENT_COUNT is set to exactly
len(CORE.component_ids) at codegen time, the StaticVector is
always correctly sized and the runtime capacity check cannot
trigger from codegen. External components that bypassed codegen
to call App.register_component() directly will now get a
compile error, forcing them to properly declare their components
in their config schema.

Co-Authored-By: J. Nick Koston <nick@koston.org>

tests/component_tests/deep_sleep/test_deep_sleep.py

@@ -8,7 +8,7 @@ def test_deep_sleep_setup(generate_main):
     main_cpp = generate_main("tests/component_tests/deep_sleep/test_deep_sleep1.yaml")
 
     assert "deepsleep = new deep_sleep::DeepSleepComponent();" in main_cpp
-    assert "App.register_component(deepsleep);" in main_cpp
+    assert "App.register_component_(deepsleep);" in main_cpp
 
 
 def test_deep_sleep_sleep_duration(generate_main):

tests/component_tests/ota/test_web_server_ota.py

@@ -27,7 +27,7 @@ def test_web_server_ota_generated(generate_main: Callable[[str], str]) -> None:
     assert "global_web_server_base" in main_cpp
 
     # Check component is registered
-    assert "App.register_component(web_server_webserverotacomponent_id)" in main_cpp
+    assert "App.register_component_(web_server_webserverotacomponent_id)" in main_cpp
 
 
 def test_web_server_ota_with_callbacks(generate_main: Callable[[str], str]) -> None:

tests/dummy_main.cpp

@@ -16,10 +16,10 @@ void setup() {
   auto *log = new logger::Logger(115200);  // NOLINT
   log->pre_setup();
   log->set_uart_selection(logger::UART_SELECTION_UART0);
-  App.register_component(log);
+  App.register_component_(log);
 
   auto *wifi = new wifi::WiFiComponent();  // NOLINT
-  App.register_component(wifi);
+  App.register_component_(wifi);
   wifi::WiFiAP ap;
   ap.set_ssid("Test SSID");
   ap.set_password("password1");