fix(tui): strengthen response status assertion in TUI A2A integration steps

Replace weak 'result is not None' check with comprehensive field validation
for tui_events and tui_output in the A2A adapter response result.

Addresses reviewer feedback from PR #10793: step_verify_response_status
was only checking result presence, not actual response content.

ISSUES CLOSED: #10793

features/steps/tui_materializer_a2a_integration_steps.py

@@ -447,4 +447,9 @@ def step_get_adapter_response(context: Any) -> None:
 @then("the response status should be set correctly")
 def step_verify_response_status(context: Any) -> None:
     """Verify the response status is set correctly."""
-    assert context.adapter_response.result is not None
+    result = context.adapter_response.result
+    assert result is not None, "Response result must not be None"
+    assert "tui_events" in result, "result must contain tui_events"
+    assert "tui_output" in result, "result must contain tui_output"
+    assert isinstance(result["tui_events"], list), "tui_events must be a list"
+    assert isinstance(result["tui_output"], str), "tui_output must be a string"