fix(kg): reject partial ISO dates to avoid silent empty result sets

Per qodo-ai review on PR #1167: sanitize_iso_date() previously accepted
YYYY and YYYY-MM, but KnowledgeGraph.query_entity() compares valid_from/
valid_to TEXT columns lexicographically against as_of. Lexicographic
comparison treats '2026-01-01' as greater than '2026' (because '-' >
end-of-string), so partial as_of values silently excluded valid facts —
re-introducing the silent-empty-results problem this PR was meant to
fix.

Tighten _ISO_DATE_RE to require YYYY-MM-DD only. Update docstring and
error message accordingly. Invert the two test cases that asserted
partials were accepted.

tests/test_config.py

@@ -223,12 +223,16 @@ def test_kg_value_rejects_over_length():
 # --- sanitize_iso_date ---
 
 
-def test_iso_date_accepts_year_only():
-    assert sanitize_iso_date("2026") == "2026"
+def test_iso_date_rejects_year_only():
+    # Partial dates re-introduce silent empty result sets via lexicographic
+    # TEXT comparison in KG queries (e.g. "2026-01-01" <= "2026" is False).
+    with pytest.raises(ValueError):
+        sanitize_iso_date("2026")
 
 
-def test_iso_date_accepts_year_month():
-    assert sanitize_iso_date("2026-03") == "2026-03"
+def test_iso_date_rejects_year_month():
+    with pytest.raises(ValueError):
+        sanitize_iso_date("2026-03")
 
 
 def test_iso_date_accepts_full_date():