gh-150449: Add tests for extreme step values in sqlite3.Blob slice operations

Add test cases for sys.maxsize and -sys.maxsize-1 step values in both
read (test_blob_get_slice_with_negative_step) and write
(test_blob_set_slice_with_negative_step) to cover the unsigned wrap-around
arithmetic path that avoids signed integer overflow.

Author: ever0de

Lib/test/test_sqlite3/test_dbapi.py

@@ -1399,6 +1399,9 @@ def test_blob_get_slice_with_negative_step(self):
         # must return b"" rather than crashing or raising an exception.
         self.assertEqual(self.blob[3:8:-1], self.data[3:8:-1])   # b""
         self.assertEqual(self.blob[5:5:-1], self.data[5:5:-1])   # b""
+        # Extreme step values: cur += (size_t)step must not overflow.
+        self.assertEqual(self.blob[5::sys.maxsize], self.data[5::sys.maxsize])
+        self.assertEqual(self.blob[::-sys.maxsize - 1], self.data[::-sys.maxsize - 1])
 
     def test_blob_set_slice(self):
         self.blob[0:5] = b"12345"
@@ -1449,6 +1452,10 @@ def test_blob_set_slice_with_negative_step(self):
         self.blob[3:8:-1] = b""
         self.assertEqual(bytes(self.blob[:]), state_before)
 
+        # Extreme step values: cur += (size_t)step must not overflow.
+        self.blob[5::sys.maxsize] = self.data[5::sys.maxsize]
+        self.blob[::-sys.maxsize - 1] = self.data[::-sys.maxsize - 1]
+
     def test_blob_mapping_invalid_index_type(self):
         msg = "indices must be integers"
         with self.assertRaisesRegex(TypeError, msg):