Fix _decode_default_value returning float for negative integer defaults

Python's str.isdigit() returns False for negative numbers like "-5",
causing DEFAULT -5 in SQLite to be decoded as -5.0 (float) instead
of -5 (int). Switch to re.fullmatch to handle the leading minus sign.

tests/test_introspect.py

@@ -309,6 +309,7 @@ def test_table_strict(fresh_db, create_table, expected_strict):
     "value",
     (
         1,
+        -5,
         1.3,
         "foo",
         True,