[FlightSQL] Statement.execute(String) can leak server prepared-statement handles when reused

### Describe the bug, including details regarding any error messages, version, and platform.

When reusing the same JDBC Statement and calling execute(String) multiple times, the Flight SQL JDBC path creates a new prepared statement on each call, but previous prepared handles are not closed before being replaced in client tracking state.

This means only the latest handle is guaranteed to be closed when the statement is closed. On stateful Flight SQL servers, this can leak server-side resources associated with older handles.

Repro
```
try (Connection connection = DriverManager.getConnection(jdbcUrl, properties);
Statement statement = connection.createStatement()) {

for (int i = 1; i <= n; i++) {
String sql = "SELECT " + i;
boolean isResultSet = statement.execute(sql);
if (isResultSet) {
try (ResultSet rs = statement.getResultSet()) {
while (rs.next()) { /* consume */ }
}
}
}
}
```

Raised in https://github.com/apache/arrow-java/pull/1090#discussion_r3132344585

Inizia dall’implementazione JDBC di Flight SQL di Statement.execute(String) e segui come vengono sostituiti e chiusi gli handle di prepared-statement; l’issue non indica alcun file né test. Riproduci il ciclo con un server Flight SQL con stato, quindi verifica che ogni handle precedente venga rilasciato prima che venga tracciato quello successivo e che Statement.close continui a ripulire l’handle finale.

Scritto dal modello di indicizzazione a partire dal testo della issue.