Update Integration Tests

[JillRegan]

The old test account is being deleted, so the integration test now points at a new vault. Matches the equivalent change in the JS SDK.

• src/onepassword/test_client.py: updated the hardcoded secret reference to the new vault and item.
• .github/workflows/validate.yml — the fork job's example step referenced TEST_SERVICE_ACCOUNT_VAULT_ID, which doesn't exist as a repo secret, so that step has never worked. Repointed it at EXAMPLE_TESTS_OP_SERVICE_ACCOUNT_TOKEN / EXAMPLE_TESTS_OP_VAULT_ID, matching the trusted job.