Fix sheet-extraction page loss: bare-list wrap, compact retry, reasoning cap

- SheetExtractorAgent accepts top-level array responses as the objects
  array instead of discarding them (recovered the failure mode behind
  16/38 failed sheets on job 475a6f184dd1)
- Second-chance compact retry per page before declaring extraction failed
- call_json: reasoning_effort param (cloud-only extra_body), finish_reason
  capture + explicit max_tokens log line, finish_reason in raw dumps,
  cache key covers reasoning_effort
- EXTRACT_MAX_TOKENS default 16384 -> 32768 (Gemini thinking tokens count
  against the cap); new EXTRACT_REASONING_EFFORT=low default for extract
- tests: 5 new fallback-ladder tests

tests/agents/test_sheet_extractor_fallback.py

@@ -0,0 +1,75 @@
+"""SheetExtractorAgent fallback ladder tests (bare-list wrap + compact retry)."""
+
+from unittest.mock import patch
+
+from backend.agents.base import AgentScope, AgentUsage
+from backend.agents.extractors import SheetExtractorAgent, _wrap_bare_list
+
+
+def _scope():
+    return AgentScope(
+        scope_id="sheet:4",
+        payload={"page": {"page_number": 4, "base64": "QUJD"}},
+    )
+
+
+def _objects(n=2):
+    return [
+        {
+            "object_id": f"obj-{i}",
+            "object_type": "equipment",
+            "category": "mechanical",
+            "name": f"RTU-{i}",
+            "source_text": f"RTU-{i}",
+            "confidence": "high",
+        }
+        for i in range(n)
+    ]
+
+
+def test_wrap_bare_list_builds_sheet_envelope():
+    wrapped = _wrap_bare_list(_objects(3), page_number=4)
+    assert wrapped["sheet"] == {}
+    assert len(wrapped["objects"]) == 3
+
+
+def test_wrap_bare_list_passes_dicts_and_none_through():
+    assert _wrap_bare_list({"sheet": {}, "objects": []}, 1) == {"sheet": {}, "objects": []}
+    assert _wrap_bare_list(None, 1) is None
+
+
+def test_run_accepts_bare_list_response():
+    agent = SheetExtractorAgent(usage=AgentUsage())
+    with patch("backend.agents.extractors.call_json",
+               return_value=_objects(5)) as mock_call:
+        result = agent.run(_scope())
+    assert not result.error
+    assert len(result.artifacts) == 1
+    sheet = result.artifacts[0]
+    assert sheet["page_number"] == 4
+    assert len(sheet["assertions"]) == 5
+    # No compact retry needed when the first call yields data.
+    assert mock_call.call_count == 1
+    # Reasoning effort knob is forwarded (None when config is blank in tests).
+    assert "reasoning_effort" in mock_call.call_args.kwargs
+
+
+def test_run_compact_retry_after_hard_failure():
+    agent = SheetExtractorAgent(usage=AgentUsage())
+    with patch("backend.agents.extractors.call_json",
+               side_effect=[None, {"sheet": {"sheet_number": "A102"},
+                                   "objects": _objects(2)}]) as mock_call:
+        result = agent.run(_scope())
+    assert not result.error
+    assert result.artifacts[0]["sheet_number"] == "A102"
+    assert mock_call.call_count == 2
+    # Second call carried the compact suffix.
+    assert "COMPACT RETRY" in mock_call.call_args_list[1].kwargs["user_text"]
+
+
+def test_run_fails_only_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"
+    assert mock_call.call_count == 2