[light] Replace initial_state storage with flash-resident callback (#15133)

tests/integration/fixtures/light_initial_state.yaml

@@ -0,0 +1,39 @@
+esphome:
+  name: light-initial-state-test
+host:
+api:  # Port will be automatically injected
+logger:
+  level: DEBUG
+
+output:
+  - platform: template
+    id: test_red
+    type: float
+    write_action:
+      - lambda: ""
+  - platform: template
+    id: test_green
+    type: float
+    write_action:
+      - lambda: ""
+  - platform: template
+    id: test_blue
+    type: float
+    write_action:
+      - lambda: ""
+
+light:
+  - platform: rgb
+    name: "Test Light"
+    id: test_light
+    red: test_red
+    green: test_green
+    blue: test_blue
+    restore_mode: ALWAYS_OFF
+    initial_state:
+      color_mode: RGB
+      state: true
+      brightness: 0.75
+      red: 1.0
+      green: 0.5
+      blue: 0.0

tests/integration/test_light_initial_state.py

@@ -0,0 +1,38 @@
+"""Integration test for light initial_state configuration.
+
+Tests that the initial_state values are correctly applied at boot when
+no saved preferences exist. The initial_state callback populates defaults
+that the restore logic uses as a fallback.
+"""
+
+import pytest
+
+from .state_utils import InitialStateHelper, require_entity
+from .types import APIClientConnectedFactory, RunCompiledFunction
+
+
+@pytest.mark.asyncio
+async def test_light_initial_state(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected: APIClientConnectedFactory,
+) -> None:
+    """Test that initial_state values are applied at boot."""
+    async with run_compiled(yaml_config), api_client_connected() as client:
+        entities, _ = await client.list_entities_services()
+        light = require_entity(entities, "test_light")
+
+        helper = InitialStateHelper(entities)
+        client.subscribe_states(helper.on_state_wrapper(lambda s: None))
+        await helper.wait_for_initial_states()
+
+        state = helper.initial_states[light.key]
+
+        # restore_mode: ALWAYS_OFF overrides state to false
+        assert state.state is False
+
+        # But the color values from initial_state should be applied
+        assert state.brightness == pytest.approx(0.75, abs=0.05)
+        assert state.red == pytest.approx(1.0, abs=0.01)
+        assert state.green == pytest.approx(0.5, abs=0.01)
+        assert state.blue == pytest.approx(0.0, abs=0.01)