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_mcp_server.py

@@ -702,14 +702,21 @@ class TestKGTools:
         assert result["success"] is False
         assert "ended" in result["error"]
 
-    def test_kg_query_accepts_partial_iso_dates(self, monkeypatch, config, palace_path, seeded_kg):
+    def test_kg_query_rejects_partial_iso_dates(self, monkeypatch, config, palace_path, seeded_kg):
         _patch_mcp_server(monkeypatch, config, seeded_kg)
         from mempalace.mcp_server import tool_kg_query
 
-        # YYYY and YYYY-MM are valid ISO-8601 forms — must not be rejected.
-        for value in ("2026", "2026-03", "2026-03-15"):
+        # Partial ISO dates are rejected: KG queries compare TEXT dates
+        # lexicographically, so "2026-01-01" <= "2026" is False, which
+        # silently excludes facts. Reject at the boundary — only YYYY-MM-DD
+        # produces correct results.
+        for value in ("2026", "2026-03"):
             result = tool_kg_query(entity="Max", as_of=value)
-            assert "error" not in result, f"rejected valid date {value!r}: {result}"
+            assert "error" in result, f"accepted partial date {value!r}: {result}"
+
+        # Full ISO-8601 dates still pass.
+        result = tool_kg_query(entity="Max", as_of="2026-03-15")
+        assert "error" not in result, f"rejected valid date: {result}"
 
 
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