[climate] Store custom mode vectors on Climate entity to eliminate heap allocation (#15206)

tests/integration/fixtures/external_components/legacy_climate_component/__init__.py

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+"""Legacy climate component — tests deprecated ClimateTraits setters backward compat.
+
+Remove this entire directory in 2026.11.0 when the deprecated setters are removed.
+"""

tests/integration/fixtures/external_components/legacy_climate_component/climate/__init__.py

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+"""Legacy climate platform that uses deprecated ClimateTraits setters."""
+
+import esphome.codegen as cg
+from esphome.components import climate
+import esphome.config_validation as cv
+from esphome.types import ConfigType
+
+legacy_climate_ns = cg.esphome_ns.namespace("legacy_climate_test")
+LegacyClimate = legacy_climate_ns.class_("LegacyClimate", climate.Climate, cg.Component)
+
+CONFIG_SCHEMA = climate.climate_schema(LegacyClimate).extend(cv.COMPONENT_SCHEMA)
+
+
+async def to_code(config: ConfigType) -> None:
+    var = await climate.new_climate(config)
+    await cg.register_component(var, config)

tests/integration/fixtures/external_components/legacy_climate_component/climate/legacy_climate.h

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+#pragma once
+
+#include "esphome/components/climate/climate.h"
+#include "esphome/core/component.h"
+
+namespace esphome::legacy_climate_test {
+
+/// Test climate that uses the DEPRECATED ClimateTraits setters for custom modes.
+/// This validates backward compatibility for external components that haven't migrated.
+class LegacyClimate : public climate::Climate, public Component {
+ public:
+  void setup() override {
+    this->mode = climate::CLIMATE_MODE_OFF;
+    this->target_temperature = 22.0f;
+    this->current_temperature = 20.0f;
+    this->publish_state();
+  }
+
+ protected:
+  climate::ClimateTraits traits() override {
+    auto traits = climate::ClimateTraits();
+    traits.add_feature_flags(climate::CLIMATE_SUPPORTS_CURRENT_TEMPERATURE);
+    traits.set_supported_modes({climate::CLIMATE_MODE_OFF, climate::CLIMATE_MODE_HEAT, climate::CLIMATE_MODE_COOL});
+    traits.set_visual_min_temperature(16.0f);
+    traits.set_visual_max_temperature(30.0f);
+    traits.set_visual_temperature_step(0.5f);
+
+    // DEPRECATED API: setting custom modes directly on ClimateTraits.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+    traits.set_supported_custom_fan_modes({"Turbo", "Silent", "Auto"});
+    traits.set_supported_custom_presets({"Eco Mode", "Night Mode"});
+#pragma GCC diagnostic pop
+
+    return traits;
+  }
+
+  void control(const climate::ClimateCall &call) override {
+    if (call.get_mode().has_value()) {
+      this->mode = *call.get_mode();
+    }
+    if (call.get_target_temperature().has_value()) {
+      this->target_temperature = *call.get_target_temperature();
+    }
+    if (call.has_custom_fan_mode()) {
+      this->set_custom_fan_mode_(call.get_custom_fan_mode());
+    }
+    if (call.has_custom_preset()) {
+      this->set_custom_preset_(call.get_custom_preset());
+    }
+    this->publish_state();
+  }
+};
+
+}  // namespace esphome::legacy_climate_test

tests/integration/fixtures/legacy_climate_compat.yaml

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+esphome:
+  name: legacy-climate-compat
+
+host:
+api:
+logger:
+  level: DEBUG
+
+external_components:
+  - source:
+      type: local
+      path: EXTERNAL_COMPONENT_PATH
+    components: [legacy_climate_component]
+
+climate:
+  - platform: legacy_climate_component
+    name: "Legacy Climate"
+    id: legacy_climate