Add integration tests for host (#8912)

tests/integration/README.md

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+# ESPHome Integration Tests
+
+This directory contains end-to-end integration tests for ESPHome, focusing on testing the complete flow from YAML configuration to running devices with API connections.
+
+## Structure
+
+- `conftest.py` - Common fixtures and utilities
+- `const.py` - Constants used throughout the integration tests
+- `types.py` - Type definitions for fixtures and functions
+- `fixtures/` - YAML configuration files for tests
+- `test_*.py` - Individual test files
+
+## How it works
+
+### Automatic YAML Loading
+
+The `yaml_config` fixture automatically loads YAML configurations based on the test name:
+- It looks for a file named after the test function (e.g., `test_host_mode_basic` → `fixtures/host_mode_basic.yaml`)
+- The fixture file must exist or the test will fail with a clear error message
+- The fixture automatically injects a dynamic port number into the API configuration
+
+### Key Fixtures
+
+- `run_compiled` - Combines write, compile, and run operations into a single context manager
+- `api_client_connected` - Creates an API client that automatically connects using ReconnectLogic
+- `reserved_tcp_port` - Reserves a TCP port by holding the socket open until ESPHome needs it
+- `unused_tcp_port` - Provides the reserved port number for each test
+
+### Writing Tests
+
+The simplest way to write a test is to use the `run_compiled` and `api_client_connected` fixtures:
+
+```python
+@pytest.mark.asyncio
+async def test_my_feature(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected: APIClientConnectedFactory,
+) -> None:
+    # Write, compile and run the ESPHome device, then connect to API
+    async with run_compiled(yaml_config), api_client_connected() as client:
+        # Test your feature using the connected client
+        device_info = await client.device_info()
+        assert device_info is not None
+```
+
+### Creating YAML Fixtures
+
+Create a YAML file in the `fixtures/` directory with the same name as your test function (without the `test_` prefix):
+
+```yaml
+# fixtures/my_feature.yaml
+esphome:
+  name: my-test-device
+host:
+api:  # Port will be automatically injected
+logger:
+# Add your components here
+```
+
+## Running Tests
+
+```bash
+# Run all integration tests
+script/integration_test
+
+# Run a specific test
+pytest -vv tests/integration/test_host_mode_basic.py
+
+# Debug compilation errors or see ESPHome output
+pytest -s tests/integration/test_host_mode_basic.py
+```
+
+## Implementation Details
+
+- Tests automatically wait for the API port to be available before connecting
+- Process cleanup is handled automatically, with graceful shutdown using SIGINT
+- Each test gets its own temporary directory and unique port
+- Port allocation minimizes race conditions by holding the socket until just before ESPHome starts
+- Output from ESPHome processes is displayed for debugging