chore(test-tests): add a BALS serialization roundtrip test

packages/testing/src/execution_testing/test_types/tests/test_block_access_list_serialization.py

@@ -0,0 +1,86 @@
+"""
+Tests for BlockAccessList serialization format.
+
+These tests verify that BAL models serialize to JSON with the correct
+format, particularly zero-padded hex strings.
+"""
+
+from execution_testing.base_types import Address, Bytes
+from execution_testing.test_types.block_access_list import (
+    BalAccountChange,
+    BalBalanceChange,
+    BalCodeChange,
+    BalNonceChange,
+    BalStorageChange,
+    BalStorageSlot,
+    BlockAccessList,
+)
+
+
+def test_bal_serialization_roundtrip_zero_padded_hex() -> None:
+    """
+    Test that BAL serializes with zero-padded hex format and round-trips correctly.
+
+    This verifies that values like 12 serialize as "0x0c" (not "0xc"), which is
+    required for consistency with other test vector fields.
+    """
+    addr = Address(0xA)
+
+    original = BlockAccessList(
+        [
+            BalAccountChange(
+                address=addr,
+                nonce_changes=[
+                    BalNonceChange(block_access_index=1, post_nonce=12),
+                    BalNonceChange(block_access_index=2, post_nonce=255),
+                ],
+                balance_changes=[
+                    BalBalanceChange(block_access_index=1, post_balance=15),
+                ],
+                code_changes=[
+                    BalCodeChange(
+                        block_access_index=3, new_code=Bytes(b"\xde\xad")
+                    ),
+                ],
+                storage_changes=[
+                    BalStorageSlot(
+                        slot=12,
+                        slot_changes=[
+                            BalStorageChange(
+                                block_access_index=1, post_value=255
+                            ),
+                            BalStorageChange(
+                                block_access_index=2, post_value=4096
+                            ),
+                        ],
+                    ),
+                ],
+                storage_reads=[1, 15, 256],
+            )
+        ]
+    )
+
+    # Serialize to JSON
+    json_data = original.model_dump(mode="json")
+    account_data = json_data[0]
+
+    # Verify zero-padded hex format (0x0c not 0xc, 0x01 not 0x1)
+    assert account_data["nonce_changes"][0]["block_access_index"] == "0x01"
+    assert account_data["nonce_changes"][0]["post_nonce"] == "0x0c"
+    assert account_data["nonce_changes"][1]["post_nonce"] == "0xff"
+    assert account_data["balance_changes"][0]["post_balance"] == "0x0f"
+    assert account_data["code_changes"][0]["block_access_index"] == "0x03"
+    assert account_data["storage_changes"][0]["slot"] == "0x0c"
+    assert (
+        account_data["storage_changes"][0]["slot_changes"][0]["post_value"]
+        == "0xff"
+    )
+    assert (
+        account_data["storage_changes"][0]["slot_changes"][1]["post_value"]
+        == "0x1000"
+    )
+    assert account_data["storage_reads"] == ["0x01", "0x0f", "0x0100"]
+
+    # Round-trip: deserialize and verify equality
+    restored = BlockAccessList.model_validate(json_data)
+    assert restored == original