Treat non-string research summaries as low quality

Filter malformed non-string research summaries instead of letting the broad exception path classify them as usable, with regression coverage.

src/research_utils.py

@@ -55,7 +55,7 @@ LOW_QUALITY_MARKERS = [
 def is_low_quality(summary: str) -> bool:
     """Check if a finding summary indicates useless or irrelevant content."""
     try:
-        if not summary:
+        if not isinstance(summary, str) or not summary:
             return True
         low = summary.lower()
         return any(marker in low for marker in LOW_QUALITY_MARKERS)

tests/test_research_utils_low_quality_nonstring.py

@@ -0,0 +1,16 @@
+from src.research_utils import is_low_quality
+
+
+def test_is_low_quality_treats_non_string_as_low_quality():
+    # Old code reached summary.lower(), hit AttributeError, and the bare
+    # except returned False (fail open) so a malformed source slipped through
+    # as "good". A non-string summary has no usable content, so it should be
+    # filtered like an empty one (which already returns True).
+    assert is_low_quality(123) is True
+    assert is_low_quality({"bad": True}) is True
+    assert is_low_quality(["does not contain"]) is True
+
+
+def test_is_low_quality_still_classifies_strings():
+    assert is_low_quality("This page does not contain relevant information") is True
+    assert is_low_quality("Detailed analysis of the 2026 EV market") is False