feat: support explicit transactions in Java sdk

[liulx20]

Summary

• add an explicit Transaction API with run, commit, rollback, close, and transaction state handling
• route transaction requests through REST endpoints with encoded transaction IDs and no automatic transport retries
• prevent session-level queries while an explicit transaction is active
• parse transaction IDs from begin-transaction responses
• document explicit transaction usage and add unit/E2E coverage

API example

try (Session session = driver.session();
        Transaction transaction = session.beginTransaction()) {
    transaction.run("CREATE (:Person {id: 1, name: 'Alice'})").close();
    transaction.commit();
}

Testing

• mvn -q test
• 64 tests passed; 3 live-server E2E tests skipped when NEUG_JAVA_E2E_URI is not set

Dependency

The driver API and unit tests can be reviewed independently. End-to-end explicit transaction behavior requires the service endpoints introduced by #873.