[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

从 Flight SQL 的 JDBC 实现中的 Statement.execute(String) 开始，跟踪 prepared-statement handles 是如何被替换和关闭的；该 issue 没有指明文件或测试。针对有状态的 Flight SQL 服务器重现该循环，然后验证每个先前的 handle 都会在下一个 handle 被跟踪之前释放，并且 Statement.close 仍会清理最终的 handle。

由索引模型根据 Issue 内容生成。