[template] Use placement new for template text restore saver (#15883)

tests/component_tests/template/config/template_text_restore.yaml

@@ -0,0 +1,14 @@
+esphome:
+  name: test
+
+host:
+
+text:
+  - platform: template
+    name: "Test Text Restore"
+    id: test_text_restore
+    optimistic: true
+    max_length: 10
+    mode: text
+    initial_value: "hello"
+    restore_value: true

tests/component_tests/template/test_template_text.py

@@ -0,0 +1,44 @@
+"""Tests for the template text component."""
+
+from __future__ import annotations
+
+from collections.abc import Callable
+from pathlib import Path
+
+
+def test_template_text_saver_uses_placement_new_with_templated_subclass(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """Regression test for template text restore saver using placement new.
+
+    When ``restore_value: true``, the saver is its own Pvariable with
+    placement new: storage is sized for ``TextSaver<MAX_LENGTH>``, the
+    declared pointer stays at ``TemplateTextSaverBase *`` for polymorphism,
+    and the templated subclass constructor runs. A regression would either
+    reintroduce the heap ``new TextSaver<...>()`` expression or size the
+    storage for the base class and silently skip the subclass ctor.
+    """
+    main_cpp = generate_main(component_config_path("template_text_restore.yaml"))
+
+    # Storage is sized and aligned for the templated subclass.
+    assert "sizeof(template_::TextSaver<10>)" in main_cpp
+    assert "alignas(template_::TextSaver<10>)" in main_cpp
+    # Pointer declared as base type for polymorphism.
+    assert (
+        "static template_::TemplateTextSaverBase *const test_text_restore_value_saver"
+        in main_cpp
+    )
+    # Placement new runs the templated subclass constructor.
+    assert "new(test_text_restore_value_saver) template_::TextSaver<10>()" in main_cpp
+    # Base-class default ctor must NOT be used.
+    assert (
+        "new(test_text_restore_value_saver) template_::TemplateTextSaverBase()"
+        not in main_cpp
+    )
+    # No heap `new TextSaver<...>()` left over — the pre-fix pattern.
+    assert "new template_::TextSaver<" not in main_cpp
+    # Saver is wired into the text component.
+    assert (
+        "test_text_restore->set_value_saver(test_text_restore_value_saver)" in main_cpp
+    )