[core] Expand = delete to catch inconvertible return types, add to_exp test

- TemplatableFn = delete now catches both stateful lambdas AND stateless
  lambdas with inconvertible return types (e.g., string -> int)
- Add test for to_exp with non-string output_type (lambda-wraps result)

tests/unit_tests/test_cpp_generator.py

@@ -684,6 +684,15 @@ async def test_templatable__with_to_exp_callable() -> None:
     assert result == 84
 
 
+@pytest.mark.asyncio
+async def test_templatable__with_to_exp_callable_and_output_type() -> None:
+    """When to_exp is provided with non-string output_type, result is lambda-wrapped."""
+    result = await cg.templatable(42, [], ct.int_, to_exp=lambda x: x * 2)
+
+    assert isinstance(result, cg.LambdaExpression)
+    assert result.capture == ""
+
+
 @pytest.mark.asyncio
 async def test_templatable__with_to_exp_dict() -> None:
     """When to_exp is a dict, value is looked up."""