[Flink] Flink catalog fails on duplicate index columns

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

When multiple Fluss indexes use the same ordered column list, the Flink connector adds all of them to the Flink schema. Flink then rejects the table with:

```text
Invalid index. There is a duplicated index composed of the same columns: [...]
```

### Solution

Update the Flink 2.2 SchemaAdapter to keep only the first index for each ordered column list. Column order remains significant, so [a, b] and [b, a] are different indexes.

### Anything else?

_No response_

### Willingness to contribute

- [x] I'm willing to submit a PR!

Locate the Flink 2.2 SchemaAdapter and trace how Fluss indexes are converted into the Flink schema. Verify that only the first index is retained for each ordered column list, while [a, b] and [b, a] remain distinct, and confirm that duplicate indexes no longer cause Flink table creation to fail.

Written by the indexing model from the issue text.