[lvgl] Fix on_value/on_update triggers for LVGL select entities (#18778)

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

tests/component_tests/lvgl/config/dropdown_update_fires_event_test.yaml

@@ -0,0 +1,36 @@
+esphome:
+  name: test-dropdown-update-event
+  on_boot:
+    - lvgl.dropdown.update:
+        id: test_dropdown
+        selected_index: 2
+
+esp32:
+  board: lolin_c3_mini
+
+spi:
+  mosi_pin:
+    number: GPIO2
+    ignore_strapping_warning: true
+  clk_pin: GPIO1
+
+display:
+  - platform: mipi_spi
+    data_rate: 20MHz
+    model: st7735
+    cs_pin:
+      number: GPIO8
+      ignore_strapping_warning: true
+    dc_pin: GPIO3
+
+lvgl:
+  widgets:
+    - dropdown:
+        id: test_dropdown
+        options:
+          - First
+          - Second
+          - Third
+        on_update:
+          - lambda: |-
+              ESP_LOGD("test", "dropdown updated");

tests/component_tests/lvgl/test_dropdown_update_fires_event.py

@@ -0,0 +1,41 @@
+"""Regression test: lvgl.dropdown.update with selected_index must fire on_value/on_update.
+
+LvSelect (backing both dropdown and roller) did not set `value_property`, so the generic
+update-action machinery in automation.py never sent the synthetic update event for a
+`selected_index:` change made via `lvgl.dropdown.update`/`lvgl.roller.update`, unlike `value:`
+on number widgets or `text:` on text widgets. Fixed by setting `LvSelect.value_property` to
+`CONF_SELECTED_INDEX`.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from esphome.__main__ import generate_cpp_contents
+from esphome.config import read_config
+from esphome.core import CORE
+
+
+@pytest.fixture(scope="module")
+def main_cpp(request: pytest.FixtureRequest) -> str:
+    config_path = (
+        Path(request.fspath).parent / "config" / "dropdown_update_fires_event_test.yaml"
+    )
+    original_path = CORE.config_path
+    try:
+        CORE.config_path = config_path
+        CORE.config = read_config({})
+        generate_cpp_contents(CORE.config)
+        return CORE.cpp_main_section
+    finally:
+        CORE.config_path = original_path
+        CORE.reset()
+
+
+def test_dropdown_update_sends_update_event(main_cpp: str) -> None:
+    assert (
+        "lv_obj_send_event(test_dropdown->obj, lvgl::lv_update_event, nullptr)"
+        in main_cpp
+    )