Fix compilation error when using string lambdas with homeassistant services (#9543)

tests/integration/fixtures/api_string_lambda.yaml

@@ -0,0 +1,64 @@
+esphome:
+  name: api-string-lambda-test
+host:
+
+api:
+  actions:
+    # Service that tests string lambda functionality
+    - action: test_string_lambda
+      variables:
+        input_string: string
+      then:
+        # Log the input to verify service was called
+        - logger.log:
+            format: "Service called with string: %s"
+            args: [input_string.c_str()]
+
+        # This is the key test - using a lambda that returns x.c_str()
+        # where x is already a string. This would fail to compile in 2025.7.0b5
+        # with "no matching function for call to 'to_string(std::string)'"
+        # This is the exact case from issue #9539
+        - homeassistant.tag_scanned: !lambda 'return input_string.c_str();'
+
+        # Also test with homeassistant.event to verify our fix works with data fields
+        - homeassistant.event:
+            event: esphome.test_string_lambda
+            data:
+              value: !lambda 'return input_string.c_str();'
+
+    # Service that tests int lambda functionality
+    - action: test_int_lambda
+      variables:
+        input_number: int
+      then:
+        # Log the input to verify service was called
+        - logger.log:
+            format: "Service called with int: %d"
+            args: [input_number]
+
+        # Test that int lambdas still work correctly with to_string
+        # The TemplatableStringValue should automatically convert int to string
+        - homeassistant.event:
+            event: esphome.test_int_lambda
+            data:
+              value: !lambda 'return input_number;'
+
+    # Service that tests float lambda functionality
+    - action: test_float_lambda
+      variables:
+        input_float: float
+      then:
+        # Log the input to verify service was called
+        - logger.log:
+            format: "Service called with float: %.2f"
+            args: [input_float]
+
+        # Test that float lambdas still work correctly with to_string
+        # The TemplatableStringValue should automatically convert float to string
+        - homeassistant.event:
+            event: esphome.test_float_lambda
+            data:
+              value: !lambda 'return input_float;'
+
+logger:
+  level: DEBUG

tests/integration/test_api_string_lambda.py

@@ -0,0 +1,85 @@
+"""Integration test for TemplatableStringValue with string lambdas."""
+
+from __future__ import annotations
+
+import asyncio
+import re
+
+import pytest
+
+from .types import APIClientConnectedFactory, RunCompiledFunction
+
+
+@pytest.mark.asyncio
+async def test_api_string_lambda(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected: APIClientConnectedFactory,
+) -> None:
+    """Test TemplatableStringValue works with lambdas that return different types."""
+    loop = asyncio.get_running_loop()
+
+    # Track log messages for all three service calls
+    string_called_future = loop.create_future()
+    int_called_future = loop.create_future()
+    float_called_future = loop.create_future()
+
+    # Patterns to match in logs - confirms the lambdas compiled and executed
+    string_pattern = re.compile(r"Service called with string: STRING_FROM_LAMBDA")
+    int_pattern = re.compile(r"Service called with int: 42")
+    float_pattern = re.compile(r"Service called with float: 3\.14")
+
+    def check_output(line: str) -> None:
+        """Check log output for expected messages."""
+        if not string_called_future.done() and string_pattern.search(line):
+            string_called_future.set_result(True)
+        if not int_called_future.done() and int_pattern.search(line):
+            int_called_future.set_result(True)
+        if not float_called_future.done() and float_pattern.search(line):
+            float_called_future.set_result(True)
+
+    # Run with log monitoring
+    async with (
+        run_compiled(yaml_config, line_callback=check_output),
+        api_client_connected() as client,
+    ):
+        # Verify device info
+        device_info = await client.device_info()
+        assert device_info is not None
+        assert device_info.name == "api-string-lambda-test"
+
+        # List services to find our test services
+        _, services = await client.list_entities_services()
+
+        # Find all test services
+        string_service = next(
+            (s for s in services if s.name == "test_string_lambda"), None
+        )
+        assert string_service is not None, "test_string_lambda service not found"
+
+        int_service = next((s for s in services if s.name == "test_int_lambda"), None)
+        assert int_service is not None, "test_int_lambda service not found"
+
+        float_service = next(
+            (s for s in services if s.name == "test_float_lambda"), None
+        )
+        assert float_service is not None, "test_float_lambda service not found"
+
+        # Execute all three services to test different lambda return types
+        client.execute_service(string_service, {"input_string": "STRING_FROM_LAMBDA"})
+        client.execute_service(int_service, {"input_number": 42})
+        client.execute_service(float_service, {"input_float": 3.14})
+
+        # Wait for all service log messages
+        # This confirms the lambdas compiled successfully and executed
+        try:
+            await asyncio.wait_for(
+                asyncio.gather(
+                    string_called_future, int_called_future, float_called_future
+                ),
+                timeout=5.0,
+            )
+        except TimeoutError:
+            pytest.fail(
+                "One or more service log messages not received - lambda may have failed to compile or execute"
+            )