fix(tests): drop removed mix_stderr kwarg from validation list CliRunner

Click 8.2 removed the mix_stderr=False keyword argument from
CliRunner.__init__. Every CliRunner(...) call in
features/steps/validation_list_command_steps.py was passing it,
causing all 7 scenarios in features/validation_list_command.feature
to error with TypeError during the @when steps. Drop the keyword
to restore the default (which now always merges stderr into stdout,
matching what the tests previously did with mix_stderr=False = True
semantics via result.output).

ISSUES CLOSED: #8621

features/steps/validation_list_command_steps.py

@@ -59,7 +59,7 @@ def _make_attachment(
 @given("a validation list command runner")
 def step_validate_list_runner(context: Any) -> None:
     """Set up a CliRunner for the validation CLI app."""
-    context.validation_list_runner = CliRunner(mix_stderr=False)
+    context.validation_list_runner = CliRunner()
     mock_service = MagicMock()
     mock_service.list_tools.return_value = []
     patcher = patch(
@@ -198,7 +198,7 @@ def step_run_list_empty(context: Any) -> None:
     """Invoke the list command with no existing validations."""
     from cleveragents.cli.main import app as main_app
 
-    runner = getattr(context, "validation_list_runner", CliRunner(mix_stderr=False))
+    runner = getattr(context, "validation_list_runner", CliRunner())
     result = runner.invoke(main_app, ["validation", "list"])
     _store_validation_list_result(context, result)
 
@@ -208,7 +208,7 @@ def step_run_list_all(context: Any) -> None:
     """Invoke the list command with default options."""
     from cleveragents.cli.main import app as main_app
 
-    runner = getattr(context, "validation_list_runner", CliRunner(mix_stderr=False))
+    runner = getattr(context, "validation_list_runner", CliRunner())
     result = runner.invoke(main_app, ["validation", "list"])
     _store_validation_list_result(context, result)
 
@@ -218,7 +218,7 @@ def step_run_list_namespace_filter(context: Any) -> None:
     """Run the list command with a --namespace filter."""
     from cleveragents.cli.main import app as main_app
 
-    runner = getattr(context, "validation_list_runner", CliRunner(mix_stderr=False))
+    runner = getattr(context, "validation_list_runner", CliRunner())
     result = runner.invoke(main_app, ["validation", "list", "--namespace", "local"])
     _store_validation_list_result(context, result)
 
@@ -228,7 +228,7 @@ def step_run_list_source_filter(context: Any) -> None:
     """Run the list command with a --source filter."""
     from cleveragents.cli.main import app as main_app
 
-    runner = getattr(context, "validation_list_runner", CliRunner(mix_stderr=False))
+    runner = getattr(context, "validation_list_runner", CliRunner())
     result = runner.invoke(main_app, ["validation", "list", "--source", "custom"])
     _store_validation_list_result(context, result)
 
@@ -238,7 +238,7 @@ def step_run_list_pattern_filter(context: Any) -> None:
     """Run the list command with a --pattern regex filter."""
     from cleveragents.cli.main import app as main_app
 
-    runner = getattr(context, "validation_list_runner", CliRunner(mix_stderr=False))
+    runner = getattr(context, "validation_list_runner", CliRunner())
     result = runner.invoke(main_app, ["validation", "list", "--pattern", "coverage.*"])
     _store_validation_list_result(context, result)
 
@@ -248,7 +248,7 @@ def step_run_list_json(context: Any) -> None:
     """Run the list command with JSON output format."""
     from cleveragents.cli.main import app as main_app
 
-    runner = getattr(context, "validation_list_runner", CliRunner(mix_stderr=False))
+    runner = getattr(context, "validation_list_runner", CliRunner())
     result = runner.invoke(main_app, ["validation", "list", "--format", "json"])
     _store_validation_list_result(context, result)
 
@@ -258,7 +258,7 @@ def step_run_list_yaml(context: Any) -> None:
     """Run the list command with YAML output format."""
     from cleveragents.cli.main import app as main_app
 
-    runner = getattr(context, "validation_list_runner", CliRunner(mix_stderr=False))
+    runner = getattr(context, "validation_list_runner", CliRunner())
     result = runner.invoke(main_app, ["validation", "list", "--format", "yaml"])
     _store_validation_list_result(context, result)