[Feature] modify column first not throws exception when column is first.

### Search before asking

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

### Motivation

Moving a field to its original position should not be considered an exceptional behavior. Therefore, when user_id is already the first column, MODIFY COLUMN user_id int FIRST should not throw an exception (MySQL does not throw an error).

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by locating the Java implementation and tests for MODIFY COLUMN, especially handling of the FIRST position. Reproduce MODIFY COLUMN user_id int FIRST when user_id is already the first column, then add coverage showing that this no-op completes without an exception.

Written by the indexing model from the issue text.