[light] Use constexpr template for DimRelativeAction transition_length (#16038)

tests/integration/fixtures/light_dim_relative_action.yaml

@@ -0,0 +1,60 @@
+esphome:
+  name: light-dim-relative-action-test
+host:
+api:
+logger:
+  level: DEBUG
+
+output:
+  - platform: template
+    id: test_out
+    type: float
+    write_action:
+      - lambda: ""
+
+light:
+  - platform: monochromatic
+    name: "Test Light"
+    id: test_light
+    output: test_out
+    default_transition_length: 0s
+
+button:
+  # Set up: turn on at 50% brightness
+  - platform: template
+    id: btn_setup
+    name: "Setup"
+    on_press:
+      - light.turn_on:
+          id: test_light
+          brightness: 50%
+
+  # Test 1: dim_relative without transition_length (HasTransitionLength=false)
+  - platform: template
+    id: btn_dim_up
+    name: "Dim Up"
+    on_press:
+      - light.dim_relative:
+          id: test_light
+          relative_brightness: 25%
+
+  # Test 2: dim_relative with transition_length (HasTransitionLength=true)
+  - platform: template
+    id: btn_dim_down
+    name: "Dim Down"
+    on_press:
+      - light.dim_relative:
+          id: test_light
+          relative_brightness: -10%
+          transition_length: 0s
+
+  # Test 3: dim_relative with brightness limits
+  - platform: template
+    id: btn_dim_clamp
+    name: "Dim Clamp"
+    on_press:
+      - light.dim_relative:
+          id: test_light
+          relative_brightness: 50%
+          brightness_limits:
+            max_brightness: 80%

tests/integration/test_light_dim_relative_action.py

@@ -0,0 +1,72 @@
+"""Integration test for light::DimRelativeAction.
+
+Tests both DimRelativeAction<HasTransitionLength=false> and
+DimRelativeAction<HasTransitionLength=true> instantiations.
+"""
+
+from __future__ import annotations
+
+import asyncio
+
+from aioesphomeapi import ButtonInfo, EntityState, LightInfo, LightState
+import pytest
+
+from .state_utils import InitialStateHelper, require_entity
+from .types import APIClientConnectedFactory, RunCompiledFunction
+
+
+@pytest.mark.asyncio
+async def test_light_dim_relative_action(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected: APIClientConnectedFactory,
+) -> None:
+    """Test light.dim_relative with and without transition_length."""
+    loop = asyncio.get_running_loop()
+    async with run_compiled(yaml_config), api_client_connected() as client:
+        light_state_future: asyncio.Future[LightState] | None = None
+
+        def on_state(state: EntityState) -> None:
+            if (
+                isinstance(state, LightState)
+                and light_state_future is not None
+                and not light_state_future.done()
+            ):
+                light_state_future.set_result(state)
+
+        async def wait_for_light_state(timeout: float = 5.0) -> LightState:
+            nonlocal light_state_future
+            light_state_future = loop.create_future()
+            try:
+                return await asyncio.wait_for(light_state_future, timeout)
+            finally:
+                light_state_future = None
+
+        entities, _ = await client.list_entities_services()
+        initial_state_helper = InitialStateHelper(entities)
+        client.subscribe_states(initial_state_helper.on_state_wrapper(on_state))
+        await initial_state_helper.wait_for_initial_states()
+
+        require_entity(entities, "test_light", LightInfo)
+
+        async def press_and_wait(name: str) -> LightState:
+            btn = require_entity(entities, name.lower().replace(" ", "_"), ButtonInfo)
+            client.button_command(btn.key)
+            return await wait_for_light_state()
+
+        # Setup: turn on at 50%
+        state = await press_and_wait("Setup")
+        assert state.state is True
+        assert state.brightness == pytest.approx(0.5, abs=0.05)
+
+        # Test 1: dim_relative without transition_length: 50% + 25% = 75%
+        state = await press_and_wait("Dim Up")
+        assert state.brightness == pytest.approx(0.75, abs=0.05)
+
+        # Test 2: dim_relative with transition_length: 75% - 10% = 65%
+        state = await press_and_wait("Dim Down")
+        assert state.brightness == pytest.approx(0.65, abs=0.05)
+
+        # Test 3: dim_relative with max_brightness limit: 65% + 50% clamped to 80%
+        state = await press_and_wait("Dim Clamp")
+        assert state.brightness == pytest.approx(0.80, abs=0.05)