fix(testing): document and harden non-AssertionError guard in apply_tdd_inversion to reduce flaky CI

Surface the non-AssertionError guard warning in standard Behave output by emitting to stderr in addition to the structured logger, and add infrastructure coverage that asserts this guard path is visible during test runs. Document the @tdd_expected_fail expectation that bug-signaling failures must use AssertionError so infrastructure exceptions are not accidentally treated as expected bug failures.

ISSUES CLOSED: #8294

features/steps/tdd_expected_fail_infrastructure_steps.py

@@ -10,6 +10,9 @@ exceptions).
 
 from __future__ import annotations
 
+from contextlib import redirect_stderr
+from io import StringIO
+
 from behave import given, then, when
 from behave.model import Scenario, Status, Step
 from behave.runner import Context
@@ -125,6 +128,19 @@ def step_run_apply_inversion_failed(context: Context) -> None:
     )
 
 
+@when(
+    "apply_tdd_inversion processes the scenario with failed True while capturing stderr"
+)
+def step_run_apply_inversion_failed_capture_stderr(context: Context) -> None:
+    """Invoke ``apply_tdd_inversion`` and capture stderr output."""
+    stderr_buffer = StringIO()
+    with redirect_stderr(stderr_buffer):
+        context.apply_inversion_result = apply_tdd_inversion(
+            context.mock_scenario, failed=True
+        )
+    context.captured_stderr = stderr_buffer.getvalue()
+
+
 @when("apply_tdd_inversion processes the scenario with failed False")
 def step_run_apply_inversion_not_failed(context: Context) -> None:
     """Invoke ``apply_tdd_inversion`` with ``failed=False``."""
@@ -151,6 +167,32 @@ def step_check_inversion_result_true(context: Context) -> None:
     )
 
 
+@then('stderr output should contain "{expected_text}"')
+def step_check_stderr_contains(context: Context, expected_text: str) -> None:
+    """Assert captured stderr output contains the expected text."""
+    assert hasattr(context, "captured_stderr"), (
+        "context.captured_stderr was not set — use the "
+        "'while capturing stderr' When step to capture stderr before asserting."
+    )
+    stderr_output: str = context.captured_stderr
+    assert expected_text in stderr_output, (
+        f"Expected stderr to contain {expected_text!r}, got: {stderr_output!r}"
+    )
+
+
+@then('stderr output should not contain "{expected_text}"')
+def step_check_stderr_not_contains(context: Context, expected_text: str) -> None:
+    """Assert captured stderr output does not contain the given text."""
+    assert hasattr(context, "captured_stderr"), (
+        "context.captured_stderr was not set — use the "
+        "'while capturing stderr' When step to capture stderr before asserting."
+    )
+    stderr_output: str = context.captured_stderr
+    assert expected_text not in stderr_output, (
+        f"Expected stderr NOT to contain {expected_text!r}, got: {stderr_output!r}"
+    )
+
+
 # ---------------------------------------------------------------------------
 # handle_tdd_expected_fail steps (standalone entry point)
 # ---------------------------------------------------------------------------