feat: coverage-driven extraction retry ladder (agent path)

tests/agents/test_sheet_extractor_fallback.py

@@ -68,9 +68,12 @@ def test_run_compact_retry_after_hard_failure():
     assert "COMPACT RETRY" in mock_call.call_args_list[1].kwargs["user_text"]
 
 
-def test_run_fails_only_after_both_attempts_miss():
+def test_run_returns_empty_sheet_after_both_attempts_miss():
     agent = SheetExtractorAgent(usage=AgentUsage())
     with patch("backend.agents.extractors.call_json", return_value=None) as mock_call:
         result = agent.run(_scope())
-    assert result.error == "no structured extraction returned"
+    # Coverage ladder: no text layer to rescue the page -> empty sheet,
+    # but no hard failure (the ladder replaced the old raise).
+    assert not result.error
+    assert result.artifacts[0]["assertions"] == []
     assert mock_call.call_count == 2