Expose Cursor.description on the Rust Postgres backend

The column names for a result set were already carried through the
cursor state machine but kept write-only, awaiting a reader. Add the
PEP-249 `Cursor.description` accessor on top of that plumbing: it
returns one 7-tuple per column (only the name populated, which is all
Synapse reads), or `None` when there is no row-returning result set —
before any query, after an error, or for a column-less statement such
as a bare INSERT, matching psycopg2.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/synapse_rust/test_database_postgres.py

@@ -493,6 +493,55 @@ class PostgresConnectionTestCase(unittest.TestCase):
 
         self.assertEqual(self.conn.run_interaction(interaction), [3, 2, 3])
 
+    # ------------------------------------------------------------------
+    # description
+    # ------------------------------------------------------------------
+
+    def test_description_is_none_before_query(self) -> None:
+        """A cursor that has not run a query has no description."""
+
+        def interaction(cursor: Any) -> Any:
+            return cursor.description()
+
+        self.assertIsNone(self.conn.run_interaction(interaction))
+
+    def test_description_reports_column_names(self) -> None:
+        """A row-returning statement describes its columns; only the name is
+        populated, in a PEP-249 7-tuple."""
+
+        def interaction(cursor: Any) -> Any:
+            cursor.execute("SELECT 1 AS a, 'x'::text AS b")
+            return cursor.description()
+
+        description = self.conn.run_interaction(interaction)
+        self.assertEqual([col[0] for col in description], ["a", "b"])
+        # Each entry is a PEP-249 7-tuple with only the name populated.
+        for col in description:
+            self.assertEqual(len(col), 7)
+            self.assertTrue(all(field is None for field in col[1:]))
+
+    def test_description_available_after_fetch(self) -> None:
+        """The description survives after the rows have been fetched."""
+
+        def interaction(cursor: Any) -> Any:
+            cursor.execute("SELECT 1 AS a")
+            cursor.fetch_all()  # exhausts the result set
+            return cursor.description()
+
+        description = self.conn.run_interaction(interaction)
+        self.assertEqual([col[0] for col in description], ["a"])
+
+    def test_description_is_none_for_dml(self) -> None:
+        """A statement that returns no rows (a bare INSERT) has no
+        description, matching psycopg2."""
+
+        def interaction(cursor: Any) -> Any:
+            cursor.execute("CREATE TEMP TABLE d (id int)")
+            cursor.execute("INSERT INTO d VALUES (1)")
+            return cursor.description()
+
+        self.assertIsNone(self.conn.run_interaction(interaction))
+
     # ------------------------------------------------------------------
     # Transaction handling (COMMIT on success, ROLLBACK on error)
     # ------------------------------------------------------------------