fix(flows): reject an explicit null step_values directive

The guard returned early on a falsy directive, so {'step_values': None}
passed validation — and because the reserved key is excluded from
ignored-key reporting, the walk then applied the caller's other fields and
returned a clean success for a directive that did nothing. Key PRESENCE is
the test, so an explicit None now falls through to the not-an-object
rejection.

Found by CodeRabbit review on #2256.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012d29UJTiH4Uy2Pm37SBPtr

Author: kingpanther13

src/ha_mcp/tools/config_entry_flow_form.py

@@ -852,9 +852,13 @@ def validate_step_values(config: dict[str, Any]) -> None:
     Accepted: a dict of ``step_id -> entry``, where an entry is a dict of field
     values, or a LIST of such dicts consumed one per encounter of that step.
     """
-    directive = config.get(_PER_STEP_VALUES_KEY)
-    if directive is None:
+    # Key PRESENCE is the test, not truthiness: an explicit ``None`` is a
+    # caller who meant to pass a directive and got the shape wrong, and the
+    # reserved key hides it from ignored-key reporting, so returning early on
+    # it would let the walk apply the rest and report a clean success.
+    if _PER_STEP_VALUES_KEY not in config:
         return
+    directive = config[_PER_STEP_VALUES_KEY]
 
     example = "{'<step_id>': {'<field>': <value>}}"
     if not isinstance(directive, dict):

tests/src/unit/test_flow_options_preserve.py

@@ -1235,8 +1235,8 @@ class TestStepValuesValidation:
 
     @pytest.mark.parametrize(
         "directive",
-        ["oops", 42, ["init"]],
-        ids=["string", "number", "list"],
+        ["oops", 42, ["init"], None],
+        ids=["string", "number", "list", "explicit-null"],
     )
     def test_a_non_object_directive_is_rejected(self, directive: Any) -> None:
         with pytest.raises(ToolError) as exc_info: