fix(search): cast query embedding to ::vector in search paper (DQ-4)

`codex search paper` crashed on the live DB with "operator does not exist:
vector <-> double precision[]". The pgvector `<->` operator needs its RHS
typed as vector; peer call sites (mcp_server.py, wiki.py) cast but the CLI
did not. Mocked unit tests never exercise real pgvector, so it went unseen.

Add the cast to both `<->` occurrences and a regression guard asserting the
SQL contains `::vector`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/cli/test_cli.py

@@ -106,6 +106,13 @@ class TestSearch:
             assert "Test Paper" in result.stdout
             assert "0.123" in result.stdout
 
+            # Regression guard (DQ-4): the pgvector `<->` operator requires the
+            # query embedding cast to ::vector. Without it, real Postgres raises
+            # "operator does not exist: vector <-> double precision[]". Mocked DBs
+            # don't catch the type error, so assert the cast is in the SQL.
+            executed_sql = mock_conn.execute.call_args_list[0][0][0]
+            assert "<->" in executed_sql and "::vector" in executed_sql
+
     def test_search_formula_command(self) -> None:
         """Test `search formula` command with mocked DB."""
         with patch("codex.db.get_conn") as mock_get_conn: