fix(plan): remove TDD expected fail tag and fix format violations

- Remove @tdd_expected_fail from Tree with json format scenario in
  plan_explain.feature (bug is now fixed by the envelope implementation)
- Update scenario assertions to verify new spec-required envelope format
  instead of old bare-array format
- Add step_tree_json_valid_envelope step definition in plan_explain_steps.py
- Fix ruff format violation in plan_explain_cli_coverage_steps.py

ISSUES CLOSED: #9163

features/plan_explain.feature

@@ -107,12 +107,12 @@ Feature: Plan explain and decision tree CLI commands
   # plan tree - json format
   # ------------------------------------------------------------------
 
-  @tdd_issue @tdd_issue_4254 @tdd_expected_fail
+  @tdd_issue @tdd_issue_4254
   Scenario: Tree with json format
     Given a set of test decisions forming a tree
     When I format the tree as json
-    Then the json tree output should be valid json
-    And the json tree output should contain "decision_id"
+    Then the json tree output should be a valid json envelope
+    And the json tree output should contain "command"
 
   # ------------------------------------------------------------------
   # plan tree - yaml format

features/steps/plan_explain_cli_coverage_steps.py

@@ -815,8 +815,12 @@ def step_pec_output_valid_json_envelope(context: Context) -> None:
         f"Expected envelope keys {_ENVELOPE_KEYS}, got {set(parsed.keys())}"
     )
     data = parsed["data"]
-    assert isinstance(data, dict), f"Expected envelope data to be a dict, got {type(data)}"
-    assert "plan_id" in data, f"Expected 'plan_id' in envelope data, got {set(data.keys())}"
+    assert isinstance(data, dict), (
+        f"Expected envelope data to be a dict, got {type(data)}"
+    )
+    assert "plan_id" in data, (
+        f"Expected 'plan_id' in envelope data, got {set(data.keys())}"
+    )
 
 
 @then("pec the tree should exclude the superseded grandchild")

features/steps/plan_explain_steps.py

@@ -405,6 +405,18 @@ def step_tree_json_valid(context: Context) -> None:
     assert isinstance(parsed, list), "Expected a JSON array"
 
 
+@then("the json tree output should be a valid json envelope")
+def step_tree_json_valid_envelope(context: Context) -> None:
+    parsed = json.loads(context.pe_tree_json)
+    assert isinstance(parsed, dict), (
+        f"Expected a JSON object (envelope), got {type(parsed)}"
+    )
+    _envelope_keys = {"command", "status", "exit_code", "data", "timing", "messages"}
+    assert _envelope_keys.issubset(parsed.keys()), (
+        f"Expected envelope keys {_envelope_keys}, got {set(parsed.keys())}"
+    )
+
+
 @then('the json tree output should contain "{text}"')
 def step_tree_json_contains(context: Context, text: str) -> None:
     assert text in context.pe_tree_json, f"Expected '{text}' in tree JSON"