[core] Fix !include vars not being substituted in !lambda values (#16320)

Co-authored-by: J. Nick Koston <nick@koston.org>
Co-authored-by: J. Nick Koston <nick@home-assistant.io>

esphome/yaml_util.py

@@ -139,7 +139,7 @@ def add_context(value: Any, context_vars: dict[str, Any] | None) -> Any:
         value.set_context({**value.vars, **(context_vars or {})})
         return value
 
-    if context_vars and isinstance(value, (dict, list, str)):
+    if context_vars and isinstance(value, (dict, list, str, Lambda)):
         value = add_class_to_obj(value, ConfigContext)
         value.set_context(context_vars)
     return value

tests/unit_tests/test_substitutions.py

@@ -818,3 +818,23 @@ def test_resolve_include_error_no_expanded_from_for_literal_filename(
         substitutions.resolve_include(include, [], substitutions.ContextVars())
 
     assert "expanded from" not in str(exc_info.value)
+
+
+def test_include_vars_applied_to_lambda_value(tmp_path: Path) -> None:
+    """!include vars: must substitute into a top-level !lambda value in the included file.
+
+    Regression test for the case where the included file's root is a Lambda;
+    add_context() previously only tagged dict/list/str, so the include's vars
+    never reached the substitution pass for Lambda content.
+    """
+    included = tmp_path / "lambda.yaml"
+    included.write_text('!lambda |-\n  return "${foo}";\n')
+
+    include = yaml_util.IncludeFile(
+        tmp_path / "main.yaml", "lambda.yaml", {"foo": "bar"}, yaml_util.load_yaml
+    )
+    config = OrderedDict({"value": include.load()})
+    result = substitutions.do_substitution_pass(config)
+
+    assert isinstance(result["value"], Lambda)
+    assert result["value"].value == 'return "bar";'