Support ALTER MATERIALIZED TABLE ... AS <query> in FlinkCatalog

### Search before asking

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

### Motivation

`FlinkCatalog#alterTable` does not support altering a materialized table's
definition query. When a user runs `ALTER MATERIALIZED TABLE AS `,
Flink produces a `TableChange.ModifyDefinitionQuery`, but `FlinkConversions`
falls through to the default branch and throws: `UnsupportedOperationException: Unsupported flink table change: ...`

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start at FlinkCatalog#alterTable and follow the FlinkConversions handling for TableChange.ModifyDefinitionQuery. Reproduce ALTER MATERIALIZED TABLE AS and trace the default branch that throws UnsupportedOperationException; done means the definition query is accepted and updated without that exception.

Written by the indexing model from the issue text.