[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

Comece pela implementação JDBC de Flight SQL de Statement.execute(String) e rastreie como os handles de prepared-statement são substituídos e fechados; a issue não nomeia nenhum arquivo nem teste. Reproduza o loop contra um servidor Flight SQL stateful e, em seguida, verifique se cada handle anterior é liberado antes que o próximo seja rastreado e se Statement.close ainda limpa o handle final.

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