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.

sqlite_utils/db.py

@@ -5214,7 +5214,7 @@ def _decode_default_value(value: str) -> object:
     if value.startswith("'") and value.endswith("'"):
         # It's a string
         return value[1:-1]
-    if value.isdigit():
+    if re.fullmatch(r"-?\d+", value):
         # It's an integer
         return int(value)
     if value.startswith("X'") and value.endswith("'"):

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,